Tunable down-hole stimulation system

ABSTRACT

Tunable down-hole stimulation systems feature closed-loop control of pumps and tunable down-hole stimulators. Stimulators generate and hydraulically transmit broad vibration spectra tuned for resonance excitation and fracturing of geologic materials adjacent to the wellbore. Feedback data for controlling stimulation includes backscatter vibration originating in stimulated geologic material and detected at the stimulator(s). For initial fracturing with relatively large particle sizes, the power spectral density (PSD) of each stimulator output is down-shifted toward the lower resonant frequencies of large particles. As fracturing proceeds to smaller (proppant-sized) fragments having higher resonant frequencies, backscatter vibration guides progressive up-shifting of stimulator PSD to higher vibration frequencies. Stimulator power requirements are minimized by concentrating vibration energy efficiently in frequency bands to which geologic materials are most sensitive at every stage of stimulation. Geologic fragmentation efficiency is thus optimized, with inherent potential for plain-water fracs completed with self-generated proppant.

FIELD OF THE INVENTION

The invention relates generally to oilfield equipment, includingreciprocating pump fluid ends and down-hole equipment for well service.Particularly demanding applications include high-pressure hydraulicstimulation on intervals along horizontal wellbores inultralow-permeability (unconventional) formations.

INTRODUCTION

New designs described herein incorporate innovative applications ofwell-known technical principles for improved stimulation systemperformance. Both adverse and beneficial aspects of the technicalprinciples are well-represented in relationships between mechanicalshocks and their associated vibration spectra. Adverse aspects of theserelationships are strikingly manifest, for example, in the troublingfailure rates of even the most modern conventional high-pressurewell-stimulation pumps. While analogous relationships can alternativelybe beneficially employed to increase pump reliability and enhance wellproductivity through more efficient and localized stimulation.

Specific examples are cited in the following paragraphs to illustratehow reliability improvements have evolved from a better understanding ofcauses and effects of shock and vibration in fluid ends. First,remarkably strong and repetitive mechanical shocks commonly originate influid ends. Second, the intensity of mechanical shocks that occur withnormal closure of conventional fluid end check valves can be reducedthrough innovative design changes. Third, without such design changes,fatigue-related damage is exacerbated, predisposing fluid ends topremature failures.

It is frequently observed that premature fluid end failures are morecommon now than they have been in the past. This is true even though theunderlying failure mechanisms have remained unchanged in some aspects.One such unchanged aspect is the fact that a mechanical shock is createdeach time a fluid end check valve closes on a valve seat. But thesemechanical shocks are generally much stronger now than they have been inthe past. The reason for a strong valve-closing shock is high backpressure, and peak back pressures are much higher now than they havebeen in the past. ***And each strong valve-closing shock inevitablygenerates high-energy broad-spectrum vibration that propagatesthroughout the fluid end.***

Broad vibration spectra, in turn, excite a range of destructiveresonances in the fluid end or the pump as a whole, predisposing variousparts to fatigue-related cracking and ultimate failure. A variety ofdesigns shown and described in the following materials explain how thesedamaging vibration resonances can be controlled (e.g., suppressed) usinga hierarchy of tunable systems, tunable subsystems, tunable componentsand design elements. Controlling destructive resonance excitation, inturn, limits vibration-induced cracking.

But resonance vibration excitation shouldn't always be limited in wellstimulation systems; sometimes it should be enhanced! Specifically,vibration spectra originating in tunable down-hole stimulators can betailored (i.e., their frequency spectra can be shifted) to maximizestimulation efficiency. Directing such (tuned) stimulation vibration togeologic material adjacent to a wellbore, in the presence of highdown-hole hydraulic pressures, typically increases rock fracturing. Thecombination of high hydraulic pressures plus tuned stimulation vibrationthus tends to both: (1) open geologic channels, and (2) prop them open,for improved well production. To optimize these complementary functions,the extent of geologic fracturing (with its associated geologicfragmentation) is periodically assessed in near-real time. Such analysisproceeds continuously as feedback data from the geologic material areprocessed in programmable controllers running empirically-derivedsoftware algorithms (broadly termed herein: frac diagnostics).

The above geologic feedback data, in the form of backscatter vibration,thus allow closed-loop control of geologic stimulation. That is,closed-loop stimulation control incorporates feedback of a portion ofthe controlled-system output (i.e., stimulated geologic material) to thecontrolled-system input (i.e., tuned stimulation vibration). Feedbackdata are used to guide re-tuning of stimulation vibration as needed,with quick convergence on optimal stimulation levels. This process isaccomplished in tunable down-hole stimulation systems via closed-loopcontrol of mechanical shocks, the shocks originating in one or moretunable down-hole stimulators.

Closed-loop control of mechanical shocks implies control of the kineticenergy impulses corresponding to a moving hammer (or mass) elementstriking, and rebounding from, a fluid interface in a tunable vibrationgenerator. At least one such generator resides within each tunabledown-hole stimulator. And at least a portion of a generator's initialkinetic energy for each hammer strike is converted to broad-spectrumvibration energy. So with each hammer strike and rebound, the vibrationspectrum's power spectral density (PSD) can be detected and adjusted asdesired under closed-loop control.

Closed-loop PSD control for tunable down-hole stimulation systems meansthat transmitted vibration spectra from a tunable vibration generatorare tuned at their source (e.g., by altering the rebound cycle time foreach hammer element strike). Such tuning effectively shapes atransmitted vibration spectrum's PSD to concentrate stimulationvibration power in predetermined frequency ranges. The predeterminedfrequency ranges for any stage of stimulation are those that maximizetransmission of vibration resonance excitation power to the adjacentgeologic materials.

The geologic materials themselves then (after a short time delay) reporttheir actual levels of resonance excitation in the form of backscattervibration feedback data. Calculated control signals based on thefeedback data then close the loop in closed-loop shock control. Thecontrol signals are calculated using a programmable controller runningfrac diagnostic software, and the signals are then applied (e.g., viafeedback control link) to one or more tunable stimulation vibrationgenerators to optimize stimulation in near-real time.

Note that evaluation of backscatter vibration data detected in one ormore down-hole stimulators may optionally be enhanced in light ofcorresponding down-hole temperature and/or hydraulic pressure datasensed at one or more down-hole stimulators. Such enhanced evaluationmay be carried out, e.g., via frac diagnostics in a programmablecontroller for the relevant tunable down-hole stimulation system.

An example benefit of such enhanced evaluation relates to estimating theeffects of down-hole hydraulic pressure changes on fluid flow (e.g.,liquid hydrocarbons) from stimulated geologic material. As stimulationproceeds, channels for such fluid flow are opened, but flow may beblocked by relatively higher down-hole hydraulic pressure. If thehydraulic pressure is sufficiently (temporarily) lowered, fluid flowfrom the stimulated geologic material ensues, carrying with it, e.g.,small particulates that may alter backscatter vibration. The vibrationalteration(s), in turn, may include Doppler shifts secondary to thevelocity of the flowing particulates, such velocity being an earlyindicator of the wellbore stage's productivity potential.

To acquire the benefits of backscatter vibration as described above,tunable down-hole stimulators transmit shock-generated stimulationvibration to geologic materials adjacent to their wellbore stages orlocation(s). Access is via, e.g., casing perforations and/or slots(i.e., ports or access openings). (See, e.g., U.S. patent applicationnumber 2014/0000909 A1, incorporated by reference). Since maximumresonance excitation frequencies necessarily change as stimulationprogresses, closed-loop shock control in the stimulator(s) causes thePSD of stimulation vibration energy to be correspondingly shifted innear-real time to optimize stimulation, and thus generate thecorresponding backscatter vibration, of individual producing zones orstages within a wellbore. (See, e.g., U.S. patent application number2014/0041876 A1, incorporated by reference). ***Optimization thus meansmore effective stimulation (i.e., more productive wells) achieved withhigher energy efficiency in less time.***

The following background materials discuss the vibration spectrum of animpulse in greater detail, highlighting its importance with examples ofthe deleterious effects of mechanical shock and vibration inconventional applications. Analogous-in-part vibration-related issues inthe automotive industry are also described to illustrate that***positive or negative aspects of vibration in mechanical systems maybecome economically important, or even evident, only at certainfrequencies and/or above certain energy levels.*** Building on thisbackground, subsequent sections describe selected alternative designsfor high-pressure pump components (e.g., check valves and vibrationdampers) and associated well-stimulation equipment (including, e.g.,tunable down-hole stimulators) which address current operational issuesof reliability, efficiency, and efficacy in well stimulation.

BACKGROUND

The necessity for modified designs (e.g., as described herein and inrelated patents) may be better appreciated after first considering: (1)the remarkably high failure rates of conventional reciprocatinghigh-pressure pumps (especially fluid ends), and (2) the substantialuncertainties (e.g., in cost/benefit analysis and technicalcomplexity/reproducibility) associated with multistage well stimulationin unconventional formations. Pump-related issues will be consideredinitially.

Frac pumps (also commonly called fracking or well-service pumps) aretypically truck-mounted for easy relocation from well-to-well. And theyare usually designed in two sections: the (proximal) power section(herein “power end”) and the (distal) fluid section (herein “fluidend”). Each pump fluid end comprises at least one subassembly (andcommonly three or more in a single fluid end housing), with eachsubassembly comprising a suction valve, a discharge valve, a plunger orpiston, and a portion of (or substantially the entirety of) a pump fluidend subassembly housing (shortened herein to “pump housing” or “fluidend housing” or “housing”, depending on the context).

For each pump fluid end subassembly, its fluid end housing comprises apumping chamber in fluid communication with a suction bore, a dischargebore, and a piston/plunger bore. A suction valve (i.e., a check valve)within the suction bore, together with a discharge valve (i.e., anothercheck valve) within the discharge bore, control bulk fluid movement fromsuction bore to discharge bore via the pumping chamber. Note that theterm “check valve” as used herein refers to a valve in which a(relatively movable) valve body can cyclically close upon a (relativelystationary) valve seat to achieve substantially unidirectional bulkfluid flow through the valve.

Pulsatile fluid flow results from cyclical pressurization of the pumpingchamber by a reciprocating plunger or piston strokes within theplunger/piston bore. Suction and pressure strokes alternately producewide pressure swings in the pumping chamber (and across the suction anddischarge check valves) as the reciprocating plunger or piston is drivenby the pump power end.

Such pumps are rated at peak pumped-fluid pressures in current practiceup to about 22,000 psi, while simultaneously being weight-limited due tothe carrying capacity of the trucks on which they are mounted. (See,e.g., U.S. Pat. No. 7,513,759 B1, incorporated by reference).

Due to high peak pumped-fluid pressures, suction check valves experienceparticularly wide pressure variations between a suction stroke, when thevalve opens, and a pressure stroke, when the valve closes. For example,during a pressure stroke with a rod load up to 350,000 pounds, aconventionally rigid/heavy check valve body may be driven longitudinally(by pressurized fluid behind it) toward metal-to-metal impact on aconventional frusto-conical valve seat at closing forces of about 50,000to over 250,000 pounds (depending on valve dimensions). A portion oftotal check-valve closure impulse energy (i.e., the total kinetic energyof the moving valve body and fluid at valve seat impact) is thusconverted to a short-duration high-amplitude valve-closure energyimpulse (i.e., a mechanical shock). As described below, each suchmechanical shock is associated with transmission of broad-spectrumvibration energy, the range of vibration spectrum frequencies being aninverse function of valve-closure energy impulse duration.

Repeated application of dual valve-closure shocks with each pump cycle(i.e., one shock from the suction valve and another shock from thedischarge valve) predisposes each check valve, and the pump as a whole,to vibration-induced (e.g., fatigue) damage. Cumulative valve-closureshocks thus significantly degrade frac pump reliability, proportional inpart to the rigidity and weight of each check valve body.

The increasing importance of fatigue-related frac pump reliabilityissues has paralleled the inexorable rise of peak pumped-fluid pressuresin new fracking applications. And insight into fatigue-related failuremodes has been gained through review of earlier shock and vibrationstudies, data from which are cited herein. For example, a recenttreatise on the subject describes a mechanical shock in terms of itsinherent properties in the time domain and in the frequency domain, andalso in terms of its effects on structures when the shock acts as theexcitation. (see p. 20.5 of Harris' Shock and Vibration Handbook, SixthEdition, ed. Allan G. Piersol and Thomas L. Paez, McGraw Hill (2010),hereinafter Harris).

References to time and frequency domains appear frequently indescriptions of acquisition and analysis of shock and vibration data.And these domains are mathematically represented on opposite sides ofequations generally termed Fourier transforms. Further, estimates of ashock's structural effects are frequently described in terms of twoparameters: (1) the structure's undamped natural frequency and (2) thefraction of critical structural damping or, equivalently, the resonantgain Q (see Harris pp. 7.6, 14.9-14.10, 20.10). (See also, e.g., U.S.Pat. No. 7,859,733 B2, incorporated by reference).

Digital representations of time and frequency domain data play importantroles in computer-assisted shock and vibration studies. In addition,shock properties are also commonly represented graphically as timedomain impulse plots (e.g., acceleration vs. time) and frequency domainvibration plots (e.g., spectrum amplitude vs. frequency). Such graphicalpresentations readily illustrate the shock effects of metal-to-metalvalve-closure, wherein movement of a check valve body is abruptlystopped by a valve seat. Relatively high acceleration values and broadvibration spectra are prominent, because each valve-closure impulseresponse primarily represents a violent conversion of a portion ofkinetic energy (of the moving valve body and fluid) to other energyforms.

Since energy cannot be destroyed, and since a conventional valve canneither store nor convert (i.e., dissipate) more than a small fractionof the valve-closure impulse's kinetic energy, a portion of that energyis necessarily transmitted to the pump housing in the form ofbroad-spectrum vibration energy. This relationship of (frequency domain)vibration energy to (time domain) kinetic energy, is mathematicallyrepresented by a Fourier transform. Such transforms are well-known tothose skilled in the art of shock and vibration mechanics. For others, agraphical representation (i.e., plots) rather than a mathematicalrepresentation (i.e., equations) may be preferable.

For example, in a time domain plot, the transmitted energy appears as ahigh-amplitude impulse of short duration. And a corresponding frequencydomain plot of transmitted energy reveals a relatively broad-spectrumband of high-amplitude vibration. ***The breadth of the vibrationspectrum is generally inversely proportional to the impulse duration.***

Thus, as noted above, a portion of the check valve's cyclicalvalve-closure kinetic energy is converted to relatively broad-spectrumvibration energy. The overall effect of cyclical check valve closuresmay therefore be compared to the mechanical shocks that would resultfrom repeatedly striking the valve seat with a commercially-availableimpulse hammer, each hammer strike being followed by a rebound. Suchhammers are easily configured to produce relatively broad-spectrumhigh-amplitude excitation (i.e., vibration) in an object struck by thehammer. (See, e.g., Introduction to Impulse Hammers athttp://www.dytran.com/img/tech/a11.pdf, and Harris p. 20.10).

Summarizing then, relatively broad-spectrum high-amplitude vibrationpredictably results from a typical high-energy valve-closure impulse.And frac pumps with conventionally-rigid valves can suffer hundreds ofthese impulses per minute. Note that the number of impulses per minute(for example, 300 impulses per minute) corresponds to pump plungerstrokes or cycles, and this number may be converted toimpulses-per-second (i.e., 300/60=5). The number 5 is sometimes termed afrequency because it is given the dimensions of cycles/second or Hertz(Hz). But the “frequency” thus attributed to pump cycles themselvesdiffers from the spectrum of vibration frequencies resulting from eachindividual pump cycle energy impulse. The difference is thatimpulse-generated (e.g., valve-generated) vibration occurs in burstshaving relatively broad spectra (i.e., simultaneously containing manyvibration frequencies) ranging from a few Hz to several thousand Hz(kHz).

In conventional frac pumps then, nearly all of the (relativelybroad-spectrum) valve-generated vibration energy must be transmitted toproximate areas of the fluid end or pump housing because vibrationenergy cannot be efficiently dissipated in the (relatively rigid) valvesthemselves. Based on extensive shock and vibration test data (seeHarris) it can be expected to excite damaging resonances that predisposethe housing to fatigue failures. (See, e.g., U.S. Pat. No. 5,979,242,incorporated by reference). If, as expected, a natural vibrationresonance frequency of the housing coincides with a frequency within thevalve-closure vibration spectrum, fluid end vibration amplitude may besubstantially increased and the corresponding vibration fatigue damagemade much worse. (See Harris, p. 1.3).

Opportunities to limit fluid end damage can reasonably begin withexperiment-based redesign to control vibration-induced fatigue. Forexample, a spectrum of vibration frequencies initially applied in theform of a test shock can reveal structural resonance frequencies likelyto cause trouble in a particular pump. These revealed frequencies areherein termed critical frequencies. For example, a test shock maycomprise a half-sine impulse of duration one millisecond, which haspredominant spectral content up to about 2 kHz (see Harris, p. 11.22).This spectral content likely overlaps, and thus will excite, a pluralityof the pump's structural resonance (i.e., critical) frequencies. Excitedcritical frequencies are then readily identified with appropriateinstrumentation, so attention can be directed to limiting operationalvibration at those critical frequencies.

Limiting vibration at critical frequencies through use of the aboveshock tests can be particularly beneficial in blocking progressivefatigue cracking in a structure. If vibration is not appropriatelylimited, fatigue cracks may grow to a point where fatigue crack size isno longer limited (i.e., the structure experiences catastrophicfracture). The size of cracks just before the point of fracture has beentermed the critical crack size. Note that stronger housings are notnecessarily better in such cases, since increasing the housing's yieldstrength causes a corresponding decrease in critical crack size (withconsequent earlier progression to catastrophic fracture). (See Harris,p. 33.23).

It might be assumed that certain valve redesigns proposed in the past(including relatively lighter valve bodies) would have alleviated atleast some of the above fatigue-related failure modes. (See, e.g., U.S.Pat. No. 7,222,837 B1, incorporated by reference). But such redesignsemerged (e.g., in 2005) when fluid end peak pressures were generallysubstantially lower than they currently are. In relatively lowerpressure applications (e.g., mud pumps), rigid/heavy valve bodiesperformed well because the valve-closure shocks and associatedvalve-generated vibration were less severe compared to shock andvibration experienced more recently in higher pressure applications(e.g., fracking). Thus, despite their apparent functional resemblance toimpulse hammers, relatively rigid/heavy valves have been pressed intoservice as candidates for use in frac pump fluid ends. Indeed, they havegenerally been among the valves most commonly available in commercialquantities during the recent explosive expansion of well-servicefracking operations. Substantially increased fluid end failure rates(due, e.g., to cracks near a suction valve seat deck) have been amongthe unfortunate, and unintended, consequences.

Under these circumstances, it is regrettable but understandable thatpublished data on a modern 9-ton, 3000-hp well-service pump includes awarranty period measured in hours, with no warranty for valves orweld-repaired fluid ends.

Such baleful vibration-related results in fluid ends might usefully becompared with vibration-related problems seen during the transition fromslow-turning two-cylinder automobile engines to higher-speed andhigher-powered inline six-cylinder engines around the years 1903-1910.Important torsional-vibration failure modes suddenly became evident inthe new six-cylinder engines, though they were neither anticipated norunderstood at the time. Whereas the earlier engines had beenunder-powered but relatively reliable, torsional crankshaft vibrationsin the six-cylinder engines caused objectionable noise (“octaves ofchatter from the quivering crankshaft”) and unexpected catastrophicfailures (e.g., broken crankshafts). (Quotation cited on p. 13 of Royceand the Vibration Damper, Rolls-Royce Heritage Trust, 2003).Torsional-vibration was eventually identified as the culprit and, thoughnever entirely eliminated, was finally reduced to a relatively minormaintenance issue after several crankshaft redesigns and the developmentof crankshaft vibration dampers pioneered by Royce and Lanchester.

Reducing the current fluid end failure rates related to valve-generatedvibration in frac pumps requires an analogous modern program ofintensive study and specific design changes. The problem will bepersistent because repeatedly-applied valve-closure energy impulsescannot be entirely eliminated in check-valve-based fluid end technology.So the valve-closing impulses must be modified, and their associatedvibrations damped, meaning that at least a portion of the totalvibration energy is converted to heat energy and dissipated (i.e., theheat is rejected to the surroundings). A reduction in total vibrationenergy results in reduced excitation of destructive resonances invalves, pump housings, and related fluid end structures. Alternatematerials, applied via innovative designs, illuminate the path forwardnow as they have in the past. Broad application of such improvementspromises higher frac pump reliability, an important near-term goal.Simultaneously, inhibition of corrosion fatigue throughout analogousfluid circuits would be advanced, a longer-term benefit in refineries,hydrocarbon crackers and other industrial venues that are also subjectedto shock-related vibration.

Further, when considering vibration in well stimulation systemscomprising frac pumps together with down-hole equipment, additionalopportunities for increased efficiency arise. Concentration ofstimulation resources near wellbore collection sites, together withfeedback-controlled application of stimulation energy, conserves timeand money. And tailoring the forms of stimulation vibration energy towell-specific geologic parameters contributes to operationalflexibility, efficiency, and efficacy.

SUMMARY OF THE INVENTION

Tunable down-hole stimulation system design includes closed-loop controlof (generally truck-mounted) pumps and tunable down-hole stimulators.Tunable down-hole stimulators hydraulically transmit broad vibrationspectra tuned for maximum resonance excitation and fracturing oftargeted geologic materials adjacent to the wellbore. Frequencies andamplitudes of both transmitted stimulation vibration and backscattervibration data are acquired by one or more detectors in each tunabledown-hole stimulator. Closed-loop control of stimulation vibration isexercised using feedback data relevant to progress of the stimulationprocess, the feedback data including backscatter vibration originatingin stimulated geologic material.

For initial (stimulation-induced) geologic fracturing, which isassociated with relatively large fragment sizes, the power spectraldensity (PSD) of tunable down-hole stimulator vibration energy isdown-shifted. That is, PSD is tuned to shift the distribution ofvibration power toward relatively lower frequencies. A PSD down-shiftthus tends to match stimulation vibration frequencies with therelatively-lower resonant frequencies of large geologic fragments. Asstimulation progresses to smaller-size fragments, corresponding (higherfrequency) backscatter vibration data are fed back (in a closed-loop) toa programmable controller. The controller then creates at least onecontrol signal and transmits it to at least one stimulator to causeprogressive up-shifting of the stimulator's transmitted vibrationenergy. That is, the PSD of the stimulator's transmitted vibrationenergy is changed by shifting the distribution of vibration power towardrelatively higher frequencies). ***Stimulator power requirements areminimized because available stimulation vibration energy is efficientlyshifted toward (and concentrated in) the bands of resonant frequenciesto which the geologic material is most susceptible at every stage ofstimulation.*** Thus, progressive geologic fragmentation at minimumenergy levels is optimized, with inherent production of self-generatedproppant as geologic fragments are progressively, and controllably,reduced to proppant-size.

Self-generated proppant may be of considerable importance if it reducesthe need for pumping of exogenous proppant (e.g., sand shipped to thedrilling site from Wisconsin). And the geologic fragmentation associatedwith self-generated proppant contributes to wellbore stimulation forincreased productivity. Both geologic stimulation and proppantgeneration are closely related to the manner in which a closed-loopstimulation system generates and modifies broad-spectrum vibrationenergy within a wellbore.

Stimulation vibration energy is generated and modified in the wellboreto minimize transmission losses as the energy travels to adjacentgeologic material. On striking the material, the energy immediatelyexcites resonant vibrations which lead to vibration-induced geologicfractures and fragmentation. Backscatter vibration data originating fromthe stimulated geologic materials is subsequently captured by detectorson one or more tunable down-hole stimulators. Analysis of thisbackscatter vibration data in near-real time then ensures thatstimulation vibration energy remains efficiently concentrated (i.e., viafeedback-controlled PSD's) for creating specific geologic responses(i.e., fractures and fragmentation), rather than being dissipated aswasteful heat. Beneficial geologic stimulation is thus obtained using acombination of minimum applied vibration energy, plus resonancevibration effects assessed via detection of backscatter vibration fromthe stimulated geologic material.

Note that plasma sources (e.g., an electric arc) are also described asproducing wide-band stimulation vibration energy. But such plasmasources do not incorporate the feedback-control of stimulationvibration, and thus the associated benefits of stimulation vibrationenergy concentration, as described herein.

The size range of geologic fragmentation encountered throughout theabove stimulation process is broad, including large chunks throughsmaller fragments to proppant-sized particles. Efficient stimulationover the entire range therefore requires current feedback data regardingfragment or particle sizes within the geologic material. (See, e.g.,U.S. Pat. Nos. 8,535,250 B2 and 8,731,848 B2, incorporated byreference). And various embodiments of the present invention provide forsuch data to be extracted substantially from band-limited backscattervibration originating in the geologic fragments themselves.

But the composition of geologic fragments demonstrates wide variations.So supplemental data may usefully be extracted (e.g., viaempirically-derived frac diagnostics) from parameters in addition tobackscatter vibration, such as ambient (down-hole) pressure andtemperature. It is particularly difficult to obtain an accuratedescription of shale reservoirs, which are substantially different fromconventional and other types of unconventional reservoirs. (See, e.g.,U.S. Pat. No. 8,731,889 B2, incorporated by reference). Thus variousembodiments of the present invention reflect a variety of differentfunctional relationships among the parameters that may bear onnecessarily time-dependent modifications of (initially broad-spectrum)stimulation vibration. The manner in which each functional relationshipamong parameters bears on vibration modifications is determined, asnoted above, by one or more frac diagnostic tools (i.e., software)residing in a programmable controller for each tunable down-holestimulation system.

Frac diagnostics reflect not only different stimulation approachesneeded for various types of geologic reservoirs, but also for differentconfigurations of tunable down-hole stimulation systems. Pertinentsystem parameters include, for example: (1) the number and type ofpumps, (2) the presence of optional pressure and/or temperature sensorsin tunable down-hole stimulators, and (3) the number of tunabledown-hole stimulators present. Notwithstanding differences among thevariety of system embodiments however, they all reflect applications ofsimilar technical principles (relating to, e.g., the vibration spectrumof an impulse) to the objective of efficient delivery of effectivedown-hole stimulation vibration.

Regarding differences mentioned above in the number and type of pumps,certain embodiments of tunable down-hole stimulation systems include oneor more pumps comprising a tunable fluid end. In such pumps,valve-generated vibration spectra are suppressed using tunablecomponents to limit fatigue-related fracturing of pump structuresaggravated by destructive excitation of pump resonances. Vibrationsuppression, as described herein, includes damping (i.e., dissipation asheat) and/or shifting of the vibration's PSD. The desired effect in eachcase is to decrease the amount of vibration power present at a pump'scritical frequencies.

Further, certain tunable down-hole stimulation system embodiments maycomprise one or more relatively higher-pressure pumps for fluid (e.g.,plain water) that contains no proppant (schematically illustrated andlabeled herein as frac pumps). One or more such frac pumps may becombined with one or more relatively lower-pressure pumps for fluidcontaining exogenous proppant (schematically illustrated and labeledherein as proppant pumps). Such system embodiments facilitate pulsedproppant placement or PPP (see below) in previously fractured geologicmaterial.

In an example scenario for initial geologic fracturing (e.g., initiallycomprising relatively large rock fragments), one or more frac pumpsprovide relatively high hydraulic down-hole pressure. Simultaneously,the PSD of broad-spectrum tunable down-hole stimulator vibration isdown-shifted (i.e., PSD is pre-tuned to shift the distribution ofstimulation vibration power toward relatively lower frequencies). Thepre-tuned vibration provides relatively lower vibration frequencies forsynergistic combination with relatively high pumped-fluid (hydraulicdown-hole) pressures to optimize initial stimulation. As stimulationprogresses, up-shifting is carried out (i.e., PSD is re-tuned to shiftthe distribution of stimulation vibration power toward relatively higherfrequencies). Such higher frequencies are needed to maintain progressivestimulation as fracturing proceeds to smaller rock fragments (e.g.,fragments approaching proppant particles in size).

As noted above, the extent of geologic fragmentation is periodicallyassessed in real time via analysis in programmable controllers runningempirically-derived algorithms (broadly termed herein: fracdiagnostics). The diagnostics operate on data including band-limitedbackscatter vibration energy from the rock fragments themselves, as wellas optionally-sensed down-hole pressure, temperature, and/or relatedparameters. The degree of rock fragmentation for each stimulation stageis thus periodically assessed for optimized flowback (e.g., throughsubstantial flow equalization within a cluster of analogous stages).

Preplanned stimulation of each fracture stage for self-generation ofproppant-sized particles is thus individually controlled via adjustmentof: (1) down-hole hydraulic pressures, and/or (2) the magnitudes andfrequencies of transmitted tunable down-hole stimulator resonanceexcitation vibrations. Inherent production of self-generated proppantfrom the stimulated geologic material is thus facilitated to supportlong-term stable productivity of each stage and cluster. Effectivenessof proppant placement, including the potential need for supplementalexogenous proppant, are assessed via comparison of transmittedstimulation vibration with its corresponding (time-displaced)backscatter vibration, in combination with associated (conventional)well logging data.

Such combined data may be used to guide intermittent (e.g., pulsed)addition of exogenous proppant to a wellbore (typically under reducedpressure), between proppant-free fracs (under relatively higherpressures). Pulsed-proppant additions minimize the total amount ofexogenous proppant needed to supplement in situ or self-generatedproppant resulting from tunable down-hole stimulation. Thus, the task ofstimulation (including proppant placement) is simplified. The overallfrac process thus becomes a series of quickly-executed incremental stepsunder closed-loop control for fast convergence on an optimal end point.

Note certain differences between pulsed-proppant placement (or PPP) asdescribed herein and the industry practice of pumping different types ofslurries or fluids in discrete intervals, that is, as slugs or stages.(See, e.g., U.S. Pat. No. 8,540,024 B2, incorporated by reference).First, proppant addition in PPP is under closed-loop control; it is afunction, in part, of backscatter vibration sensed down-hole innear-real time by one or more detectors on each tunable down-holestimulator. Second, proppant-laden fluid may be injected into a wellbore(via a separate proppant pump) at lower pressures than those associatedwith the frac pump(s) which otherwise feed the wellbore with aproppant-free fluid stream. And Third, proppant provided via the PPPclosed-loop system is supplemental to self-generated proppant which iscreated anew through stimulation vibration transmitted by one or moretunable down-hole stimulators.

A tunable down-hole stimulation system embodiment to accomplish such PPPis schematically illustrated herein to emphasize certain advantagesstemming from separation of the relatively high-pressure frac pump fromthe (optionally) relatively lower-pressure proppant pump. The absence ofproppant in the relatively high-pressure fluid end means longer life forhigh-pressure valves, with resultant improvements in fluid endreliability. Even in proppant pumps, valve life-cycles and fluid-endreliability would improve because the valves would (optionally) beoperating at relatively lower pressures, thereby producing relativelyless-energetic valve-generated vibration with narrowed (and lessdamaging) vibration spectra.

As described herein, control of vibration spectra (particularly theirpower spectral densities or PSD's) guides the design and operation ofboth tunable fluid ends and tunable down-hole stimulators. Thesesubsystems, in turn, contribute to the increased reliability andproductivity of tunable down-hole stimulation systems. Fundamentalprinciples are invoked throughout this description to explain majorbenefits of improved vibration suppression (in tunable fluid ends) andimproved vibration generation, transmission and analysis (via tunabledown-hole stimulators and programmable system controllers).

In the following paragraphs, both generation of broad-spectrum vibrationin tunable down-hole stimulators, and incorporation of the stimulatorsin tunable down-hole stimulation systems, are considered before controlof valve-generated vibration in tunable fluid ends. This is to emphasizethe role of induced resonance excitation vibration in geologic materialsfor maximizing well productivity.

Suppression of resonance excitation in tunable fluid ends, on the otherhand, limits the destructive effects of valve-generated vibration (formaximizing fluid end reliability). Comparisons will be noted between therelated-in-part techniques for inducing or suppressing a desired rangeof resonance-related power spectral densities in systems comprising bothtunable down-hole stimulators and (optionally) tunable fluid ends.

The desirability of tunable down-hole stimulators in tunable down-holestimulation systems stems from the well-known vertical and horizontalheterogeneity of unconventional reservoirs. Wide variability of geologicmaterials adjacent to wellbores is common, meaning thatconsistently-beneficial stimulation design has been difficult toachieve. In current practice, some fracture stages are typically foundto be substantially more productive than others, while the cost offracking varies little from stage-to-stage. Thus, stimulation designcurrently reflects compromises between the efficiency of a singlecustomized fracture stage and the degraded performance of multipleone-size-fits-all stages that include a variety of geologic materialshaving different productive potentials.

Such currently unavoidable inefficiencies are substantially reduced bythe advent of new tunable down-hole stimulation systems as describedherein. With the new systems, progressive series of customized fracturestages can be realized in near-real time through productive integrationof: (1) pumps (optionally having tunable fluid ends), (2) tunabledown-hole stimulators, and (3) programmable controllers. Each fracturestage is electively customized in turn, through use of near-real timefrac diagnostics. Relatively productive stages can be readily identifiedfor optimal stimulation, followed by combination of such stages intostrategically important productive clusters.

And the twin keys to creation of productive clusters in horizontalwellbores are (1) gathering backscatter vibration data generated indifferent portions of a tunable down-hole stimulation system and (2)processing these and related data (e.g., pressure and/or temperature) inthe system's programmable controller to create control signals. Controlsignals, in turn, optimize stimulation; they can also facilitateaccurate placement and adjustment of inflow control devices within atunable down-hole stimulation system.

Embodiments of tunable down-hole systems may comprise a plurality oftunable subsystems, e.g., (optionally) tunable fluid ends and/or tunabledown-hole stimulators. And each subsystem may comprise tunablecomponents, e.g., tunable valves, tunable valve seats, and/or tunablevibration generators. Functions of tunable subsystems are coordinatedvia control elements including, for example, down-hole pressure and/ortemperature sensors, detectors for both transmitted and backscattervibration, and programmable computers running various frac diagnosticsoftware programs. Communication pathways (e.g., the control linksschematically illustrated herein) connect control elements, as well astunable components and subsystems.

Control links carry information (e.g., data, electronic signals foranalog or digital encoded variables, and parameters such as frequency,time, amplitude, pressure values, and/or temperature values).Communication pathways include, e.g., mechanical links, electricalcables, wireless links, and/or fluid-filled spaces. While exampleembodiments of tunable subsystems, tunable components, control elementsand control links are described and schematically illustrated herein,the following examples are intended to be merely representative ofcertain functional groupings to enhance the clarity of overall systemdescription.

One such functional grouping concerns detection, analysis andmodification of vibration. In either closed-loop or open-loopembodiments, a single (wide dynamic range) accelerometer can function asa detector of vibration, whether transmitted as broad-spectrumstimulation energy or received in the form of band-limited backscattervibration energy. But since the typical energy levels of (relativelyhigh-energy) transmitted vibration and (relatively low-energy)backscatter vibration may differ substantially, two or more separateaccelerometers may be preferred for greater accuracy. Regardless oftheir arrangement in a tunable down-hole stimulation system, one or morevibration detectors initiate the flow of feedback information necessaryfor closed-loop control.

An illustration of such closed-loop control is seen in a firstembodiment of a tunable down-hole stimulation system. The systemcomprises at least one frac pump for creating down-hole hydraulicpressure, together with at least one tunable down-hole stimulator, eachstimulator comprising a tunable vibration generator for transmittingvibration hydraulically. The system further comprises a programmablecontroller for creating a plurality of control signals and transmittingat least one control signal to each frac pump and each tunable down-holestimulator. Additionally, each tunable down-hole stimulator comprises atleast one accelerometer for sensing vibration and for transmitting anelectrical signal derived therefrom (i.e., for transmitting anelectrical signal which is a function of the vibration as sensed by theaccelerometer through change in one or more accelerometer electricalparameters such as capacitance, inductance and/or capacitance). And theprogrammable controller is responsive to that electrical signal (i.e.,the programmable controller creates at least one control signal as afunction of that electrical signal).

Each tunable down-hole stimulator comprises a hammer elementlongitudinally movable within a hollow cylindrical housing having alongitudinal axis, a first end, and a second end, the first end beingclosed by a fluid interface, and the second end being closed by a driverelement. The driver element comprises at least one field emissionstructure for moving the hammer element to strike, and rebound from, thefluid interface to generate broad-spectrum vibration.

Each of the above field emission structures is responsive to at leastone control signal, and the generated broad-spectrum vibration has apredetermined power spectral density responsive to at least one controlsignal. Additionally, each tunable vibration generator has acharacteristic rebound frequency, and each accelerometer is responsiveto that characteristic rebound frequency.

The characteristic rebound frequency is substantially influenced by thedriver element. And each driver element may comprise one or moremagnetic field emission structures and/or one or more electric fieldemission structures. A hammer element (i.e., a mass) is longitudinallymovable within the cylindrical housing between the driver element andthe fluid interface. Such movement is influenced (i.e., controlled in anopen-loop or closed-loop manner) by forces exerted on the hammer via themagnetic and/or electrical fields of the field emission structure(s).(See, e.g., U.S. Pat. No. 8,760,252 B2, incorporated by reference). Tofacilitate hammer element movement, the hammer element may comprise,e.g., one or more permanent magnets, and the driver element's fieldemission structure(s) may comprise, e.g., one or more electromagnets, atleast one with reversible polarity and variable field strength. See the'252 patent for other examples of field emission structures.

By design, the hammer element periodically moves toward impact on thefluid interface (analogous in part to the impact of a fluid end valveclosure), followed by movement away from the fluid interface (i.e.,rebounding from the impact). More specifically, the hammer element moves(under the influence of the driver element's electric and/or magneticfields) to strike, and rebound from, the fluid interface, thusgenerating broad-spectrum vibration. In other words, the cylindricalhousing, driver element, hammer element, and fluid interface functiontogether as a tunable vibration generator for stimulation.

Note that the hammer element is responsive to the driver element bothfor striking, and rebounding from, the fluid interface. Responsivenessof the hammer element may be achieved via open-loop control (usingempirically-derived predictions of hammer element direction and velocitybased, e.g., on field emission strength) or closed-loop control (using,e.g., feedback data on hammer element position to calculate directionand velocity of hammer element movement). The latter data may beobtained, e.g., via an electric field sensor on the fluid interfaceinteracting with an electret electric field emission structure on thehammer element.

Note further that a block diagram of a tunable down-hole stimulator isschematically illustrated herein as having an internal stimulatorinterconnect which connects two sections: (1) a section comprising anoptional down-hole pressure sensor with a tunable vibration generator asdescribed above and (2) a section comprising one or more optionaltemperature sensors combined with one or more detectors for transmittedand backscatter vibration. The separate (and more numerous) functions ofthese two sections with their detectors and optional sensors may becompared with the internal structural features shown in the separateschematic illustration of a tunable hydraulic stimulator. Takentogether, the two schematic illustrations and the written descriptionherein explain how the relatively less-complex tunable hydraulicstimulator might be used in a relatively-simple open-loop system, whilethe functionally more-complex tunable down-hole stimulator is adapted tothe greater demands of a closed-loop feedback control system.

Regardless of a stimulator's configuration, stimulation vibration energymay preferably be transmitted from down-hole stimulators in relativelyshort bursts that are spaced apart in time. Time-delayed backscattervibration energy may then be sensed at the same or different down-holestimulators in the periods between bursts of transmitted vibration. Butboth transmitted and backscatter vibration energy can be detected at thefluid interface because they will be present at different times. Thus,one or more accelerometers may provide data on both transmitted andbackscatter vibration energy, as well as on the delay time inherent inbackscatter vibration.

Delay time, in turn, may be interpreted (e.g., using frac diagnostics)to indicate the stimulation depth or total distance traveled by thebackscatter vibration energy. Further, changes in the backscattervibration's power spectral density (see below) may also (again usingfrac diagnostics) be used to characterize the geologic material along awellbore. Thus, vibration information detected by one or more detectorsat a fluid interface, as well as estimates of related parameters (e.g.,Doppler shift) that can be extracted therefrom, may be particularlyuseful when determining the preferred directions, depths and lengths ofmultiple wellbores to be placed in a relatively confined geologic space.

The importance of vibration information is reflected in the schematicillustration herein of a tunable down-hole stimulation system. Theillustration includes a block-diagram of a tunable down-hole stimulator,the diagram clearly separating the function of transmittingbroad-spectrum vibration for stimulation from the functions of detectingboth transmitted vibration energy and band-limited (and time-shifted)backscatter vibration energy. The separated functions emphasize, forexample, that changes in backscatter vibration's frequency band limitsare reflected as shifts in the vibration energy's power spectral density(or PSD) plot. That is, an up-shift in PSD will reveal that relativelylower frequencies represent a smaller fraction of the plot's totalvibration energy. And relatively higher frequencies will be seen torepresent a greater portion of the plot's total vibration energy. Suchan up-shift could occur naturally as stimulation of geologic materialprogresses, with backscatter vibration arising in ever-smallerstimulated particles having relatively higher resonant frequencies.

Since backscatter vibration emanates from particles experiencingvibration resonance excitation (i.e., stimulation), changes in thebackscatter vibration's PSD can reveal changes in the particles'resonance frequencies. And since particles' resonance frequencies arefunctions of, among other things, particle size and composition (e.g.,hardness), analysis of PSD data can directly indicate the local effectsof stimulation. In other words, frac diagnostics applied during thestimulation process can provide near real-time information on thechanging nature of the stimulated geologic material. ***Specifically,the extent and range of stimulation-generated fragmentation can beestimated through analysis of sequential PSD shifts in band-limitedbackscatter vibration energy.***

Note that the influence of absolute power levels on backscattervibration calculations may be substantially reduced through scaling ofpower measurements (including PSD) to local maxima.

Note also that periodic estimates of the degree of shift in PSD may beused to estimate progress (in near-real time) toward a desired end pointfor stimulation. Thus, stimulation may be optimized via control ofvibration energy to achieve a predetermined degree of fragmentation. Ifmore fragmentation is desired, one may up-shift (or re-tune) the powerspectral density of the originally-transmitted vibration to make moretotal high-frequency stimulation energy available. A tunable down-holestimulator facilitates this up-shift through responsiveness of itshammer element's rebound cycle time to its driver element's fieldemission structure(s).

Responsiveness of a hammer element to a driver element of a tunabledown-hole stimulator may be achieved via, e.g., a field emissionstructure comprising an electromagnet/controller having programmablemagnetic field polarity reversal and variable magnetic field strength,as seen, e.g., in linear reversible motors. Control of magnetic fieldstrength is optionally via open-loop and/or closed-loop networksassociated with the electromagnet/controller. Note that such magneticfield strength control allows the driver to influence hammer elementmovement before, during and after each impact via attractive orrepelling spatial forces. See. e.g., the '252 patent for a discussion ofspatial forces.

In practice, cyclical changes in magnetic field strength may becharacterized by a polarity reversal frequency responsive to theaccelerometer signal mentioned earlier and/or to a signal from a tunabledown-hole stimulator system controller. Longitudinal movement of thehammer element is thus responsive in part (e.g., via electromagneticattraction and repulsion) to the driver element's cyclical magneticpolarity reversal. For example, longitudinal movement of the hammerelement striking, and subsequently rebounding from, the fluid interfacemay be substantially in phase with the polarity reversal frequency togenerate vibration transmitted by the fluid interface.

Thus, each hammer strike is at least in part a function of magneticfield polarity and strength, and it is followed by a rebound which is atleast in part a function of flexure due to elastic properties (e.g.,modulus of elasticity) of the hammer and fluid interface. The reboundmay also be a function of the driver element's magnetic field polarityand strength. The duration of the hammer element's entireflexure-rebound cycle is thus controllable; it is termed herein “hammerrebound cycle time” and is measured in seconds. The inverse of hammerrebound cycle time has the same dimensions as frequency (e.g., cyclesper second) and is termed “characteristic rebound frequency” herein.

Each hammer strike & rebound applies a mechanical shock to the fluidinterface which generates a (relatively-broad) spectrum of stimulationvibration frequencies that are transmitted hydraulically via the fluidinterface (and the surrounding down-hole fluid) to the adjacent geologicmaterial. (See the Background section above). The breadth of thegenerated stimulation vibration spectrum is a reflection of a mechanicalshock's duration (i.e., the rebound cycle time). Shortening the reboundcycle time broadens the generated-vibration spectrum (i.e., the spectrumextends to include relatively higher frequencies). The power spectraldensity is therefore up-shifted, meaning that more of the total power ofthe transmitted spectrum is represented in the higher frequencies. Inthis manner, additional stimulation energy (i.e., rock-fracturingenergy) may be directed to relatively smaller rock fragments becausethese fragments have resonances at the relatively-higher stimulationvibration frequencies. Thus, a tunable down-hole stimulator'stransmitted stimulation vibration energy may be controlled so as toencourage continued geologic fragmentation to a predetermined fragmentsize (e.g., to a size for effective function as a proppant).

Summarizing, hammer rebound movement may be either augmented or impededby the driver element's magnetic field polarity and strength, therebychanging hammer rebound cycle time and thus changing the character ofstimulation vibration spectra generated. That is, the driver element'sfield emission structure comprising an electromagnet/controller caneffectively, and in near-real time, tune each stimulation vibrationspectrum transmitted by the fluid interface for application to geologicmaterial adjacent to a wellbore. Such tuning may comprise, for example,altering a transmitted vibration spectrum's bandwidth and/or changingthe relative magnitudes of the vibration spectrum's frequency components(i.e., changing the spectrum's power spectral density). In other words,stimulation energy in the form of vibration spectra transmitted by atunable down-hole stimulator's fluid interface may be subject (innear-real time) to alterations in response to ongoing results of fracdiagnostic calculations operating on feedback data inherent inbackscatter vibration.

Note that alternative embodiments of a down-hole stimulation vibrationgenerator may be described as having the form of a linear electricalmotor, the hammer element acting as an armature. One such form is seenin railguns, with the armature providing the conducting connectionbetween (parallel) rails. In this case, opposing currents in the rails(and thus the hammer movement) would be controlled by the driver toachieve the desired characteristic rebound frequency. (See, e.g., U.S.Pat. Nos. 8,371,205 B2 and 8,677,877 B2, both incorporated byreference).

Progressive alterations in the character of stimulation vibration energyapplied to down-hole geologic material may include, for example, changesin vibration frequencies present, changes in relative energy levels ofvibration frequency components, and/or changes in the total power of apulse of stimulation vibration. Such changes may be desirable whilestimulation proceeds through a continuum of fracturing of the geologicmaterial. Progress of stimulation is reflected in backscatter vibration,the character of which changes with continued fracturing and/orfragmentation of the geologic material. That is, the geologic material'sabsorption of stimulation vibration energy, and its radiation ofbackscatter vibration, changes in a time-varying manner. Changes inabsorbed stimulation energy, in turn, cause changes in backscattervibration that may be detected (e.g., by an accelerometer) at the fluidinterface. The resulting accelerometer signal may then be fed back tothe driver (e.g., by cable or wireless link) as described herein.

The invention thus facilitates a form of closed-loop (feedback) controlof the stimulation process that may be optimized (i.e., to yield betterresults from less stimulation). One might choose, for example, toemphasize relatively higher down-hole pressure and lower frequencystimulation vibration energy initially. One might then choose toadaptively decrease the down-hole pressure and increase relativelyhigher frequency stimulation vibration spectrum components asstimulation progresses. Individual tunable down-hole stimulators of theinvention can support such an optimization strategy inherently becausethey naturally produce relatively broad vibration spectra (rather thansubstantially single-frequency vibration like an aviation black-boxpinger). Should a greater frequency range be desired than thatobtainable from a single tunable down-hole stimulator, a plurality ofsuch stimulators may be interconnected in a ***tunable down-holestimulator array***. Operation of such an array may be controlled via,for example, a programmable controller as part of a tunable down-holestimulation system, the controller having one or more control links tothe driver of each stimulator of a tunable down-hole stimulator array.Such an array may reveal location dependent resonances through cross PSDcalculations in the programmable controller. (See, e.g., U.S. Pat. No.8,571,829 B2, incorporated by reference).

In such a tunable down-hole stimulator array, the driver elementpolarity reversal frequency of each down-hole stimulator may be maderesponsive to a band-limited portion of backscatter vibrationrepresented in one accelerometer signal. Alternatively or additionally,stimulation vibration of the array may be subject to control viaprogrammable devices elsewhere in a wellbore and/or at the wellhead.

A second embodiment of tunable down-hole stimulation system is similarin several respects to the above first embodiment, comprising at leastone frac pump for creating down-hole hydraulic pressure. But the secondembodiment differs in that at least one frac pump is combined with atleast one proppant pump connected in parallel with the frac pump foradding exogenous proppant. The system further comprises at least onetunable down-hole stimulator, each stimulator comprising a tunablevibration generator having a characteristic rebound frequency. Aprogrammable controller is included for creating a plurality of controlsignals and transmitting at least one control signal to each frac pump,each proppant pump, and each tunable down-hole stimulator. Each tunabledown-hole stimulator comprises at least one accelerometer for sensingvibration and for transmitting an electrical signal derived therefrom(i.e., for transmitting an electrical signal which is a function of thevibration as sensed by the accelerometer through change in one or moreaccelerometer electrical parameters such as capacitance, inductanceand/or resistance). And each accelerometer is responsive to thecharacteristic rebound frequency (i.e., the accelerometer transmits atleast one electrical signal which is a function of the characteristicrebound frequency). Finally, the programmable controller is responsiveto the electrical signal (i.e., the programmable controller creates atleast one control signal as a function of that electrical signal).Further, longitudinal hammer element movement, as noted above, isassociated with the tunable vibration generator's characteristic reboundfrequency. In certain embodiments, the characteristic rebound frequencymay be similar to the magnetic field's polarity reversal frequency toaid control of the hammer's longitudinal displacement.

A third embodiment of a tunable down-hole stimulation system differsfrom the above first and second embodiments in part because it comprisesa wellbore which itself comprises a vertical wellbore, a kickoff point,a heel, and a toe. At least one frac pump creates down-hole hydraulicpressure in the wellbore, and at least one tunable down-hole stimulatoris located within the wellbore (and between the heel and toe). Eachstimulator comprises a tunable vibration generator, and a programmablecontroller creates a plurality of control signals and transmits at leastone control signal to each frac pump and each tunable down-holestimulator. Each tunable down-hole stimulator comprises at least oneaccelerometer for sensing vibration and for transmitting an electricalsignal derived therefrom, and the programmable controller is responsiveto the electrical signal.

As noted above, part of the vibration sensed at the fluid interfacetypically includes time-delayed backscatter vibration. It also maycontain temperature data related to the degree of rock fracturing and/orfragmentation, including the size of rock fragments. Fracturing-relatedtemperature changes may be induced in part by mechanical inefficienciessecondary to vibration earlier transmitted from the fluid interface.(See U.S. Pat. No. 8,535,250 B2, incorporated by reference). Hence,temperature-related well-stimulation data can be used to augment controlof fracturing resulting from transmitted stimulation vibration.

Note also that the driver element's polarity and field strength may alsoor alternatively be directly responsive (e.g., via integrated controlelectronics and windings of the electromagnet) to the magnetic fieldstrength at the fluid interface. Such responsiveness may be mediated bychanges in the magnetic field permeability sensed by the controlelectronics, the permeability changes being in part functions of theamplitude and frequency of backscatter vibration received by the fluidinterface. Reception of the backscatter vibration thus allows (by atleast two mechanisms) near-real-time estimation of the degree ofstimulation imposed by the tunable down-hole stimulator.

An important determinant of imposed stimulation is the hammer element'sstriking face, which has a predetermined modulus of elasticity that maybe relatively high (approximately that of mild steel, for example) if arelatively broad spectrum of stimulation vibration is desired.Conversely, a lower modulus of elasticity may be chosen to reduce thehighest frequency components of stimulation vibration spectra. Forconvenience, alternate hammer embodiments may comprise one or aplurality of interchangeable striking faces, each having a single valuewithin a predetermined range of modulus choices. Choice of that rangewill facilitate tuning of the down-hole stimulator to predeterminedstimulation vibration spectra, of course, and the range of vibrationspectra parameters (e.g., frequency and magnitude) will also beinfluenced by the fluid interface's modulus of elasticity and the designcriteria vibration spectrum frequency range.

The spectra of stimulation vibration desired for a particularapplication will generally be chosen to encompass one or more of the(estimated) resonant frequencies of the geologic structures beingstimulated (including resonant frequencies before, during, and afterstimulation). For example, it has been reported that vibrationfrequencies in the ultrasound range (i.e., >20 kHz) can improve thepermeability of certain porous media surrounding a well. On the otherhand, vibration frequencies <20 kHz may propagate with less loss, whilestill significantly increasing well flow rates. (See, e.g., U.S. patentpublication number 2014/0027110 A1, incorporated by reference).Optimization of the stimulation process may be facilitated usingestimates obtained via (1) one or more programmable microprocessors inthe tunable down-hole stimulator and/or (2) one or more programmablemicroprocessors in the tunable down-hole stimulation system controller.Such estimates may be based in part, e.g., on the portion(s) of thebackscatter vibration energy from stimulated porous media.

Note that a tunable down-hole stimulator is intended for down-hole usewithin a fluid environment maintained in the wellbore via (1) fluidscollected through explosively-formed perforations or preformed slots inthe wellbore casing from the surrounding geologic formations and/or via(2) addition of fluid at the wellhead to equal or exceed the filtrationrate (sometimes termed the leakoff rate). (See U.S. Pat. No. 8,540,024B2, incorporated by reference). The fluid surrounding a stimulator maycomprise water and/or petroleum oil, and it may be passively pressurizedby the well's hydraulic head alone, or with additional pressure providedby one or more frac pumps. Since either the tunable down-hole stimulatoror the tunable hydraulic stimulator can be completely sealed frominternal contact with surrounding fluid, use of either embodiment is notsubject to dielectric strength and conductivity limitations (e.g.,“compensation dielectric liquid” required in U.S. patent publicationnumber 2014/0027110 A1 cited above) that are common in pulsed powerapparatus. (See also U.S. Pat. No. 8,616,302 B2, incorporated byreference).

Note also that tunable resilient circumferential seals are electivelyprovided to isolate predetermined explosively-formed perforations orpreformed slots in portions of the wellbore casing (analogous in part toswell packers). (See, e.g., U.S. patent application number 2014/0051612A1, incorporated by reference). Such tunable seals can provide a tunedcoupling of the stimulator to the wellbore casing. The circumferentialseal comprises a circular tubular area which may contain at least oneshear-thickening fluid to assist tuning to a preferred frequency range.And the fluid may further comprise nanoparticles which, in conjunctionwith the shear-thickening fluid, also facilitate tuning of the seal aswell as heat scavenging.

Having summarized certain improvements in new tunable down-holestimulation systems as a whole, this description now focuses onsubsystems and components (e.g., structures and functions of the fracpump's fluid end and down-hole stimulators). While a tunable down-holestimulator is intended to generate vibration to augment fracturing ofrock formations, the focus in fluid ends is on control ofvalve-generated vibration for minimizing excitation of fluid end and/orpump resonances to avoid fatigue-mediated fluid end failures.

Tunable fluid ends reduce valve-generated vibration to increasefluid-end reliability. Tunable fluid end embodiments comprise a family,each family member comprising a pump housing with at least one installedtunable component chosen from: tunable check valve assemblies, tunablevalve seats, tunable radial arrays and/or tunable plunger seals. Eachtunable component, in turn, contributes to blocking excitation of fluidend resonances, thus reducing the likelihood of fluid end failuresassociated with fatigue cracking and/or corrosion fatigue. Bydown-shifting the frequency domain of each valve-closing impulse shock,initial excitation of fluid end resonances is minimized. Subsequentdamping and/or selective attenuation of vibration likely to excite oneor more predetermined (and frequently localized) fluid end resonancesrepresents optimal employment of vibration-control resources.

Frequency domain down-shifting and damping both assist vibration controlby converting valve-closure energy to heat and dissipating it in eachtunable component present in a tunable fluid end embodiment. Effects ofdown-shifting on a valve-closure impulse shock includefrequency-selective spectrum-narrowing that is easily seen in thefrequency domain plot of each shock. That is, down-shifting effectivelyattenuates and/or limits the bandwidth(s) of valve-generated vibration.Subsequent (coordinated) damping assists in converting a portion of thisband-limited vibration to heat.

Both down-shifting and damping are dependent in part on constraintscausing shear-stress alteration (that is, “tuning”) imposed on one ormore viscoelastic and/or shear-thickening materials in each tunablecomponent. Additionally, hysteresis or internal friction (see Harris, p.5.7) associated with mechanical compliance of certain structures (e.g.,valve bodies or springs) may aid damping by converting vibration energyto heat (i.e., hysteresis loss). (See Harris, p. 2.18).

Tunable component resonant frequencies may be shifted (or tuned) toapproximate predetermined values corresponding to measured or estimatedpump or fluid end housing resonant frequencies. Such coordinated tuningpredisposes valve-generated vibration at critical frequencies to excitethe tunable component (and thus be damped and dissipated as heat) ratherthan exciting the housing itself (and thus predispose it to vibrationfatigue-related cracking).

To complement the above coordinated damping, frequency down-shiftingfunctions to reduce the total amount of critical frequency vibrationrequiring damping. Such down-shifting is activated through designsenhancing mechanical compliance. In continuous pump operation,mechanical compliance is manifest, for example, in elastic valve bodyflexures secondary to repetitive longitudinal compressive forces (i.e.,plunger pressure strokes). Each such flexure is followed by ahysteresis-limited elastic rebound, the duration of the entireflexure-rebound interval being termed herein “rebound cycle time.” Theinverse of rebound cycle time is termed herein “characteristic reboundfrequency.” Cumulative rebound cycle energy loss in the form of heat(e.g., hysteresis loss plus friction loss) is continuously transportedfor redistribution within the valve body and eventual rejection to thevalve body surroundings (including, e.g., the pumped fluid). This heatloss represents a reduction in the available energy content (and thusthe damage-causing potential) of the valve-closure energy impulse.

Note that lengthening rebound cycle time to beneficially narrow thevalve-generated vibration spectrum is accomplished in various inventionembodiments using mechanical/hydraulic/pneumatic analogs of electronicwave-shaping techniques. For example, lengthened rebound cycle time issubstantially influenced by the tunable valve assembly's increasedlongitudinal compliance associated with rolling seal contact (i.e.,comprising valve body flexure and rebound) described herein between thevalve body's peripheral valve seat interface and the tunable valveseat's mating surface.

Briefly summarizing, as each tunable component present in a tunablefluid end embodiment absorbs, converts and redistributes (i.e.,dissipates) a portion of valve closing impulse shock energy, only afraction of the original closing impulse energy remains at criticalfrequencies capable of exciting destructive resonant frequencies in thefluid end. Following vibration down-shifting, a significant portion ofvalve-closure energy has been shifted to lower frequency vibrationthrough structural compliance as described above. This attenuatedvibration is then selectively damped (i.e., dissipated as heat) atshifted frequencies via one or more of the tunable components. Whiletunable components may be relatively sharply tuned (e.g., to act astuned mass dampers for specific frequencies), they may alternately bemore broadly tuned to account for a range of vibration frequenciesencountered in certain pump operations. Flexibility in tuningprocedures, as described herein with material and adjustment choices, istherefore desirable.

Note that vibration absorption at specific frequencies (e.g., viadynamic or tuned absorbers) may have limited utility in frac pumpsbecause of the varying speeds at which the pumps operate and therelatively broad bandwidths associated with valve-closing impulseshocks. In contrast, the process of down-shifting followed by damping ismore easily adapted to changes inherent in the pumps' operationalenvironment. Damping may nevertheless be added to a dynamic absorber toincrease its effective frequency range for certain applications. (See,e.g., tuned vibration absorber and tuned mass damper in ch. 6 ofHarris).

Selective damping of vibration frequencies near the resonant frequenciesof fluid ends is desirable for the same reason that soldiers break stepwhen they march over a bridge—because even relatively small amounts ofvibration energy applied at the bridge's resonant frequency can causecatastrophic failure. Similar reasoning underlies the functions ofselective vibration down-shifting and damping in tunable fluid ends.Various combinations of the tunable components described herein areparticularly beneficial because they focus the functions ofvibration-limiting resources on minimization of vibration energy presentin a fluid end near its housing's critical frequencies. Cost andcomplexity of tunable components are thus minimized while the efficacyof each tunable component's function (i.e., vibration limitation atparticular frequencies) is enhanced. Stated another way, a tunablecomponent's selective vibration down-shifting and damping are optimizedusing metrics including cost, complexity, and damping factor (or degreeof damping).

Note that a variety of optimization strategies for vibration attenuationand damping may be employed in specific cases, depending on parameterssuch as the Q (or quality) factor attributable to each fluid endresonance. The fluid end response to excitation of a resonance may berepresented graphically as, for example, a plot of amplitude vs.frequency. Such a Q response plot typically exhibits a single amplitudemaximum at the local fluid end resonance frequency, with decreasingamplitude values at frequencies above and below the resonance. At anamplitude value about 0.707 times the maximum value (i.e., thehalf-power point), the amplitude plot corresponds not to a singlefrequency but to a bandwidth between upper and lower frequency values oneither side of the local fluid end resonance. The quality factor Q isthen estimated as the ratio of the resonance frequency to the bandwidth.(See, e.g., pp. 2-18, 2-19 of Harris). (See also U.S. Pat. No. 7,113,876B2, incorporated by reference).

Lower Q connotes the presence of more damping and a wider bandwidth(i.e., a relatively broader band of near-resonant frequencies). Andhigher Q connotes less damping and a narrower bandwidth (ideally, zerodamping and a single resonant frequency). Since ideal fluid endresonances are not encountered in practice, optimization strategiestypically include choice of the peak resonant frequency and Q of thetunable component in light of the peak resonant frequency and Q of thefluid end resonance of interest. Tunable component resonant frequenciesidentified herein as “similar” to fluid end or pump housing resonancesare thus understood to lie generally in the frequency range indicated bythe upper and lower frequency values of the relevant Q responsehalf-power bandwidth.

In tunable components of the invention, choice of Q depends on bothmaterials and structure, especially structural compliances and theproperties of viscoelastic and/or shear-thickening materials present inthe component(s). Further, the peak (or representative) frequency of atunable component or a fluid end resonance may not be unambiguouslyobtainable. Thus, optimization of tunable component vibration dampingmay be an iterative empirical process and may not be characterized by asingle-valued solution. Note also that tunable component resonantfrequencies may be intentionally “detuned” (i.e., adjusted to slightlydifferent values from nominal resonant or peak frequencies) in pursuitof an overall optimization strategy.

To minimize fluid end fatigue failures then, resonant frequencies ofeach tunable component of the invention are adjusted (i.e., tuned) usinganalytical and/or empirical frequency measures. Such measures areconsidered in light of the resonant frequencies of any other tunablecomponent(s) present, and also in light of critical resonances of thefluid end or pump itself. The objective is optimal attenuation anddamping of the most destructive portion(s) of valve-generated vibration.In each case, optimal vibration limitation will be dependent on thecomponent's capacity to dissipate heat generated through hysteresis,friction and/or fluid turbulence. Thus, certain predetermined portion(s)of valve-closure energy are dissipated at one or more predetermined pumphousing resonant (critical) frequencies.

Note that the critical frequencies proximate to a fluid end suction boremay differ, for example, from the critical frequencies proximate to thesame fluid end's plunger bore due to the different constraints imposedby structures proximate the respective bores. Such differences areaccounted for in the adjustment of tunable components, particularlytunable valve seats and tunable plunger seals.

What follows are descriptions of the structure and function of eachtunable component that may be present in a tunable fluid end embodiment,the fluid end having at least one fluid end resonant frequency. Eachtunable fluid end embodiment comprises at least one subassembly, andeach subassembly comprises a housing (e.g., a fluid end housing or pumphousing with appropriate bores). Within each housing's respective boresare a suction valve, a discharge valve, and a plunger or piston. When atunable fluid end comprises multiple subassemblies (which is the generalcase), the respective subassembly housings are typically combined in asingle fluid end housing. And at least one subassembly has at least onetunable component. In specific tunable fluid end embodiments, tunablecomponents may be employed singly or in various combinations, dependingon operative requirements.

The first tunable component described herein is a tunable check valveassembly (one being found in each tunable check valve). Installed in afluid end for high pressure pumping, a tunable check valve assemblycomprises at least one vibration damper or, in certain embodiments, aplurality of (radially-spaced) vibration dampers disposed in a valvebody. Each vibration damper constitutes at least one tunable structuralfeature. Since the fluid end has at least a first fluid end resonancefrequency, at least one vibration damper has (i.e., is tuned to) atleast a first predetermined assembly resonant frequency similar to thefirst fluid end resonance (i.e., resonant frequency). If, for example,the fluid end has a second fluid end resonance frequency (a commonoccurrence), a single vibration damper and/or at least one of aplurality of vibration dampers may have (i.e., be tuned to) at least asecond predetermined assembly resonant frequency similar to the secondfluid end resonance frequency. In general, the specific manner ofdamping either one or a plurality of fluid end resonance frequencieswith either one or a plurality (but not necessarily the same number) ofvibration dampers is determined during the optimization process notedabove.

Each of the sample embodiments of tunable check valve assembliesschematically illustrated herein comprises a check valve body havingguide means (to maintain valve body alignment during longitudinalmovement) and a peripheral valve seat interface. A peripheral groovespaced radially apart from a central reservoir is present in certainembodiments, and a viscoelastic structure may be present in theperipheral groove (i.e., the groove damping element). In one suchembodiment, the assembly's vibration dampers comprise a plurality ofradially-spaced viscoelastic body structures disposed in the groove andreservoir, the viscoelastic groove element comprising a groove circulartubular area. In alternative embodiments, the viscoelastic reservoir (orcentral) damping element may be replaced by a central spring-massdamper. A viscoelastic central damper may be tuned, for example, via aflange centrally coupled to the valve body. A spring-mass central dampermay be tuned, for example, by adjusting spring constant(s) and/ormass(es), and may also or additionally be tuned via the presence of aviscous or shear-thickening liquid in contact with one or more damperelements.

A reservoir (or central) damping element tuning frequency may be, asnoted above, a first predetermined assembly resonant frequency similarto a first fluid end resonance. Analogously, the groove circular tubulararea may comprise at least one shear thickening material providing themeans to tune the groove damping element to at least a secondpredetermined assembly resonant frequency similar, for example, toeither a first or second fluid end resonant frequency. The choice oftuning frequencies for the reservoir and groove damping elements is notfixed, but is based on a chosen optimization strategy for vibrationdamping in each fluid end.

Note that phase shifts inherent in the (nonlinear) operation of certainvibration dampers described herein create the potential for a pluralityof resonant frequencies in a single vibration damper.

Note also that the longitudinal compliance of a tunable check valveassembly affects its rebound cycle time and thus influences vibrationattenuation (i.e., downshifting or spectrum narrowing), whichconstitutes a form of tuning. Further, vibration dampers in alternativetunable check valve assembly embodiments may comprise spring-masscombinations having discrete mechanical components in addition to, or inplace of, viscoelastic and/or shear-thickening components. An example ofsuch a spring-mass combination within a valve body central reservoir isschematically illustrated herein.

The second tunable component described herein is a tunable valve seat,certain embodiments of which may be employed with a conventional valvebody or, alternatively, may be combined with a tunable check valveassembly to form a tunable check valve. A tunable valve seat in a fluidend for high pressure pumping comprises a concave mating surface and/ora lateral support assembly longitudinally spaced apart from a matingsurface. A lateral support assembly, when present, is adjustably secured(e.g., on a lateral support mounting surface) or otherwise coupled tothe mating surface. A lateral support assembly is a tunable structuralfeature for resiliently coupling the tunable valve seat to a fluid endhousing (and thus damping vibrations therein). That is, a lateralsupport assembly (and thus a tunable valve seat of which it is a part)has at least one tunable valve seat resonant frequency similar to atleast one fluid end resonant frequency. Further, a lateral supportassembly may be combined with a concave mating surface to provide twotunable structural features in a single tunable valve seat. Tunabilityof the concave mating surface inheres in its influence on rebound cycletime through the predetermined orientation and degree of curvature ofthe concave mating surface. Since it constitutes a tunable structuralfeature, a concave mating surface may be present in a tunable valve seatwithout a lateral support assembly. In the latter case, the concavemating surface will be longitudinally spaced apart from a pump housinginterface surface, rather than a lateral support mounting surface(examples of these two surfaces are schematically illustrated herein).In light of a tunable valve seat's potential for embodying either one ortwo tunable structural features, a plurality of tunable valve seatresonant frequencies may characterize a single tunable valve seat, withthe respective frequencies being chosen in light of the fluid endresonance(s) and the valve closure impulse vibration spectrum.

Flexibility in the choice of tunable valve seat resonant frequencies isguided by optimization criteria for vibration control in a tunable fluidend. Such criteria will suggest specifics of a lateral supportassembly's structure and/or the concave curvature of a mating surface.For example, a support assembly's one or more suitably-secured circularviscoelastic support elements comprise a highly adaptable supportassembly design for resiliently coupling the tunable valve seat to afluid end housing (and thus damping vibrations therein). At least onesuch viscoelastic support element comprises a support circular tubulararea. And each support circular tubular area, in turn, comprises atleast one shear thickening material having (i.e., being tuned to aresonance frequency similar to) at least one seat resonant frequencythat may be chosen to be similar to at least one fluid end resonantfrequency. As above, the choice of tuning frequency or frequencies for atunable valve seat is not fixed, but is based on a predeterminedoptimization strategy for vibration damping in each fluid end

Note that in addition to individual tuning of a tunable check valveassembly and a tunable valve seat (forming a tunable check valve), thecombination may be tuned as a whole. For example, a tunable check valvein a fluid end for high pressure pumping may alternatively oradditionally be tuned for spectrum narrowing by ensuring that itscharacteristic rebound frequency (i.e., a function of rebound cycletime) is less than at least one fluid end resonant frequency. In such acase, for example, at least one tunable valve seat resonant frequencymay be similar to at least one fluid end resonant frequency.

The third tunable component described herein is a tunable radial arraydisposed in a valve body. In a schematically illustrated embodiment, thevalve body comprises guide means, a peripheral valve seat interface, anda fenestrated peripheral groove spaced radially apart from a centralreservoir. A viscoelastic body element disposed in the groove (thegroove element) is coupled to a viscoelastic body element disposed inthe reservoir (the reservoir element) by a plurality of viscoelasticradial tension members passing through a plurality of fenestrations inthe peripheral groove. Each radial tension member comprises at least onepolymer composite and functions to couple the groove element with thereservoir element, a baseline level of radial tension typically arisingdue to shrinkage of the viscoelastic elements during curing. Thetensioned radial members, as schematically illustrated herein, assistanchoring of the coupled groove element firmly within the peripheralseal-retention groove without the use of adhesives and/or serrations ashave been commonly used in anchoring conventional valve seals. Radialtension members also create a damped resilient linkage of groove elementto reservoir element (analogous in function to a spring-mass damperlinkage). This damped linkage can be “tuned” to approximate (i.e., havea resonance similar to) one or more critical frequencies via choice ofthe viscoelastic and/or composite materials in the damped linkage. Notethat radial tension members also furnish a transverse preload force onthe valve body, thereby altering longitudinal compliance, rebound cycletime (and thus characteristic rebound frequency), and vibrationattenuation.

The fourth tunable component described herein is a tunable plunger sealcomprising at least one lateral support assembly (analogous to that of atunable valve seat) securably and sealingly positionable along aplunger. Typically, a lateral support assembly will be installed in apacking box (sometimes termed a stuffing box) or analogous structure.The tunable plunger seal's lateral support assembly is analogous instructure and function to that of a tunable valve seat, as are thetuning procedures described above.

Note that the predetermined resonant frequency of each circularviscoelastic element of a lateral support assembly is affected by theviscoelastic material(s) comprising it, as well as by constraintsimposed via adjacent structures (e.g., portions of a valve seat, fluidend housing, packing box or plunger). The choice of a variety ofviscoelastic element inclusions includes, for example, reinforcingfibers, circular and/or central cavities within the viscoelasticelement, and distributions of special-purpose materials (e.g.,shear-thickening materials and/or graphene) within or in associationwith one or more viscoelastic elements.

Note also that the lateral support assembly of either a tunable valveseat or a tunable plunger seal resiliently links the respective valveseat or plunger with adjacent portions of a fluid end housing,effectively creating a spring-mass damper coupled to the housing. Thisdamped linkage can be “tuned” to approximate one or more criticalfrequencies via, e.g., shear-thickening materials in the respectivecircular tubular areas as described herein.

Analogous damped linkages between the housing and one or more auxiliarymasses may be incorporated in tunable fluid end embodiments forsupplemental vibration damping at one or more fluid end resonantfrequencies (e.g., auxiliary tuned vibration absorbers and/or tuned-massdampers). Additionally or alternatively, one or more damping surfacelayers (applied, e.g., as metallic, ceramic and/or metallic/ceramiccoatings) may be employed for dissipating vibration and/or for modifyingone or more fluid end resonant frequencies in pursuit of an overalloptimization plan for fluid end vibration control. Such damping surfacelayers may be applied to fluid ends by various methods known to thoseskilled in the art. These methods may include, for example, cathodicarc, pulsed electron beam physical vapor deposition (EB-PVD), slurrydeposition, electrolytic deposition, sol-gel deposition, spinning,thermal spray deposition such as high velocity oxy-fuel (HVOF), vacuumplasma spray (VPS) and air plasma spray (APS). The surface layers may beapplied to the desired fluid end surfaces in their entirety or appliedonly to specified areas. Each surface layer may comprise a plurality ofsublayers, at least one of which may comprise, for example, titanium,nickel, cobalt, iron, chromium, silicon, germanium, platinum, palladiumand/or ruthenium. An additional sublayer may comprise, for example,aluminum, titanium, nickel, chromium, iron, platinum, palladium and/orruthenium. One or more sublayers may also comprise, for example, metaloxide (e.g., zirconium oxide and/or aluminum oxide) and/or anickel-based, cobalt-based or iron-based superalloy. (See e.g., U.S.Pat. No. 8,591,196 B2, incorporated by reference).

Further as noted above, constraints on viscoelastic elements due toadjacent structures can function as a control mechanism by alteringtunable component resonant frequencies. Examples of such effects areseen in embodiments comprising an adjustable flange coupled to the valvebody for imposing a predetermined shear preload by further constraininga viscoelastic element already partially constrained in the reservoir.One or more tunable check valve assembly resonant frequencies may thusbe predictably altered. Consequently, the associated valve-generatedvibration spectrum transmissible to a housing may be narrowed, and itsamplitude reduced, through hysteresis loss of valve-closure impulseenergy at each predetermined assembly resonant frequency (e.g., byconversion of valve-closure impulse energy to heat energy, rather thanvibration energy).

In addition to composite viscoelastic element inclusions, controlmechanisms for alteration of tunable component resonant frequenciesfurther include the number, size and spacing of peripheral groovefenestrations. When fenestrations are present, they increase valveassembly responsiveness to longitudinal compressive force whilestabilizing viscoelastic and/or composite peripheral groove elements.Such responsiveness includes, but is not limited to, variations in thewidth of the peripheral groove which facilitate “tuning” of the groovetogether with its viscoelastic element(s).

Briefly summarizing, each embodiment of a tunable component attenuatesand/or damps valve-generated vibration at one or more fluid end criticalfrequencies. The transmitted vibration spectrum is thus narrowed and itsamplitude reduced through conversion and dissipation of valve-closureimpulse (kinetic) energy as heat. One or more tunable componentstructural features are thus tunable to one or more frequencies similarto at least one fluid end resonant frequency to facilitateredistribution/dissipation of impulse kinetic energy, following itsconversion to heat energy.

Continuing in greater detail, valve-closure impulse energy conversion ina tunable component primarily arises from hysteresis loss (e.g., heatloss) in viscoelastic and/or discrete-mechanical elements, but may alsooccur in related structures (e.g., in the valve body itself). Hysteresisloss in a particular structural feature is related in-part to thatfeature's compliance (i.e., the feature's structural distortion as afunction of applied force).

Compliance arises in structural features of a tunable component, such asone or more viscoelastic elements, plus at least one other compliantportion. For example, a tunable check valve body distorts substantiallyelastically under the influence of a closing energy impulse, and itsassociated viscoelastic element(s) simultaneously experience(s) shearstress in accommodating the distortion. The resulting viscoelastic shearstrain, however, is at least partially time-delayed. And the time delayintroduces a phase-shift useful in damping valve-generated vibration(i.e., reducing its amplitude). Analogous time-delay phase shift occursin a mass-spring damper comprising discrete mechanical elements.

In addition to vibration damping, a complementary function of a tunablecomponent is narrowing of the spectrum of valve-generated vibration.Spectrum narrowing (or vibration down-shifting) is associated withcompliance in the form of deformation over time in response to anapplied force. Since each instance of compliance takes place over afinite time interval, the duration of a closing energy impulse iseffectively increased (and the vibration spectrum correspondinglynarrowed) as a function of compliance.

A narrowed valve-generated vibration spectrum, in turn, is less likelyto generate destructive sympathetic vibration in adjacent regions of afluid end housing. For this reason, compliant portions of a valve bodyare designed to elastically distort under the influence of the closingenergy impulse (in contrast to earlier substantially-rigid valvedesigns). Compliance-related distortions are prominent in, but notlimited to, the shapes of both the (peripheral) groove and the(relatively central) reservoir. Viscoelastic elements in the groove andreservoir resist (and therefore slow) the distortions, thus tending tobeneficially increase the closing energy impulse's duration whilenarrowing the corresponding vibration spectrum.

Distortions of both groove and reservoir viscoelastic body elementsresult in viscoelastic stress and its associated time-dependent strain.But the mechanisms differ in the underlying distortions. In a peripheralgroove, for example, proximal and distal groove walls responddifferently to longitudinal compressive force on the tunable check valveassembly. They generally move out-of-phase longitudinally, therebyimposing time-varying compressive loads on the groove viscoelasticelement. Thus the shape of the groove (and the overall compliance of thegroove and its viscoelastic element) changes with time, making thegroove as a whole responsive to longitudinal force on the assembly.

Peripheral groove fenestrations increase groove responsiveness tolongitudinal force. As schematically illustrated herein, fenestrationsincrease groove responsiveness by changing the coupling of the proximalgroove wall to the remainder of the valve body (see Detailed Descriptionherein).

In the reservoir, in contrast, responsiveness to longitudinal force maybe modulated by an adjustable preload flange centrally coupled to thevalve body. The flange imposes a shear preload on the viscoelasticreservoir element (i.e., shear in addition to that imposed by thereservoir itself and/or by the closing energy impulse acting on theviscoelastic element via the pumped fluid). The amount of shear preloadvaries with the (adjustable) radial and longitudinal positions of theflange within the reservoir. The overall compliance and resonances ofthe reservoir and its viscoelastic element may be predictably altered bysuch a shear preload, which is imposed by the flange's partialconstraint of the viscoelastic reservoir element. Note that whenreservoir and groove viscoelastic body elements are coupled by aplurality of radial tension members, as in a tunable radial array, theradial tension members lying in groove wall fenestrations allowtransmission of shear stress between the groove and reservoirviscoelastic elements.

Thus, in tunable radial array embodiments, at least a firstpredetermined resonant frequency may substantially replicate a (similar)pump housing resonant frequency via adjustment of shear preload on thereservoir viscoelastic element. The plurality of fenestration elementscoupling the reservoir element with the groove element may have at leasta second predetermined resonant frequency related to the firstpredetermined resonant frequency and optionally achieved through choiceof tensile strength of the radial tension members (i.e., fenestrationelements). And at least a third predetermined resonant frequency relatedto the first and second predetermined resonant frequencies may beachieved through choice of at least one shear thickening material incircular tubular areas of the groove viscoelastic element and/or one ormore support circular tubular areas.

Note that any structural feature of a tunable check valve assembly ortunable radial array (e.g., a valve body or a viscoelastic element) maybe supplemented with one or more reinforcement components to form acomposite feature. Reinforcement materials tend to alter compliance andmay comprise, for example, a flexible fibrous material (e.g., carbonnanotubes, graphene), a shear-thickening material, and/or othermaterials as described herein.

As noted above, alterations in compliance (with its associatedhysteresis loss) contribute to predetermined vibration spectrumnarrowing. Such compliance changes (i.e., changes in displacement as afunction of force) may be achieved through adjustment of constraint.Constraint, in turn, may be achieved, e.g., via compression appliedsubstantially longitudinally by the adjustable preload flange to aconstrained area of the viscoelastic reservoir element. In embodimentscomprising a central longitudinal guide stem, the constrained area maybe annular. And adjacent to such an annular constrained area may beanother annular area of the viscoelastic reservoir element which is notin contact with the adjustable preload flange (i.e., an annularunconstrained area). This annular unconstrained area is typically opento pumped fluid pressure.

Preload flange adjustment may change the longitudinal compliance of thetunable check valve assembly by changing the effective flange radiusand/or the longitudinal position of the flange as it constrains theviscoelastic reservoir element. Effective flange radius will generallyexceed actual flange radius due to slowing of (viscous) viscoelasticflow near the flange edge. This allows tuning of the check valveassembly to a first predetermined assembly resonant frequency formaximizing hysteresis loss. Stated another way, by constraining avibrating structure (e.g., an area of the viscoelastic reservoirelement), it is possible to force the vibrational energy into differentmodes and/or frequencies. See, e.g., U.S. Pat. No. 4,181,027,incorporated by reference.

The invention thus includes means for constraining one or more separateviscoelastic elements of a valve assembly, as well as means forconstraining a plurality of areas of a single viscoelastic element. Andsuch constraint may be substantially constant or time-varying, withcorrespondingly different effects on resonant frequencies. Peripherally,time-varying viscoelastic element constraint may be provided byout-of-phase longitudinal movement of peripheral groove walls. Incontrast, time-varying viscoelastic element constraint may be appliedcentrally by a flange coupled to the valve body.

Flange radial adjustment is facilitated, e.g., via a choice amongeffective flange radii and/or flange periphery configurations (e.g.,cylindrical or non-cylindrical). Flange longitudinal movement may beadjusted, for example, by (1) use of mechanical screws or springs, (2)actuation via pneumatic, hydraulic or electrostrictive transducers, or(3) heat-mediated expansion or contraction. Flange longitudinal movementmay thus be designed to be responsive to operational pump parameterssuch as temperature, acceleration, or pressure. Since pump housingresonant frequencies may also respond to such parameters, tunable checkvalve assemblies and tunable check valves may be made at least partiallyself-adjusting (i.e., operationally adaptive or auto-adjusting) so as tochange their energy-absorbing and spectrum-narrowing characteristics tooptimally extend pump service life.

Note that in certain embodiments, the preload flange may comprise asubstantially cylindrical periphery associated with substantiallylongitudinal shear. Other embodiments may comprise a non-cylindricalperiphery for facilitating annular shear preload having bothlongitudinal and transverse components associated with viscoelastic flowpast the flange. Such an invention embodiment provides for damping oftransverse as well as longitudinal vibration. Transverse vibration mayoriginate, for example, when slight valve body misalignment with a valveseat causes abrupt lateral valve body movement during valve closing.

Note also that one or more flanges may or may not be longitudinallyfixed to the guide stem for achieving one or more predetermined assemblyresonant frequencies.

And note further that the first predetermined assembly resonantfrequency of greatest interest, of course, will typically approximateone of the natural resonances of the pump and/or pump housing.Complementary hysteresis loss and vibration spectrum narrowing may beadded via a second predetermined assembly resonant frequency achievedvia the viscoelastic groove element (which may comprise at least onecircular tubular area containing at least one shear-thickeningmaterial). The time-varying viscosity of the shear-thickeningmaterial(s), if present, furnishes a non-linear constraint of thevibrating structure analogous in part to that provided by the adjustablepreload flange. The result is a predetermined shift of the tunable checkvalve assembly's vibrating mode analogous to that described above.

Note that when a nonlinear system is driven by a periodic function, suchas can occur with harmonic excitation, chaotic dynamic behavior ispossible. Depending on the nature of the nonlinear system, as well asthe frequency and amplitude of the driving force, the chaotic behaviormay comprise periodic oscillations, almost periodic oscillations, and/orcoexisting (multistable) periodic oscillations and nonperiodic-nonstabletrajectories (see Harris, p. 4-28).

In addition to a shift in the tunable check valve assembly's vibratingmode, incorporation of at least one circular tubular area containing atleast one shear-thickening material within the viscoelastic grooveelement increases impulse duration by slightly slowing valve closure dueto reinforcement of the viscoelastic groove element. Increased impulseduration, in turn, narrows the closing energy impulse vibrationspectrum. And shear-thickening material itself is effectivelyconstrained by its circular location within the viscoelastic grooveelement(s).

The shear-thickening material (sometimes termed dilatant material) isrelatively stiff near the time of impact and relatively fluid at othertimes. Since the viscoelastic groove element strikes a valve seat beforethe valve body, complete valve closure is slightly delayed by theshear-thickening action. The delay effectively increases thevalve-closure energy impulse's duration, which means that vibrationwhich is transmitted from the tunable check valve assembly to its(optionally tunable) valve seat and pump housing has a relativelynarrower spectrum and is less likely to excite vibrations thatpredispose a pump housing to early fatigue failure. The degree ofspectrum narrowing can be tuned to minimize excitation of known pumphousing resonances by appropriate choice of the shear-thickeningmaterial. Such vibration attenuation, and the associated reductions inmetal fatigue and corrosion susceptibility, are especially beneficial incases where the fluid being pumped is corrosive.

The functions of the viscoelastic groove element, with its circularshear-thickening material, are thus seen to include those of aconventional valve seal as well as those of a tunable vibrationattenuator and a tunable vibration damper. See, e.g., U.S. Pat. No.6,026,776, incorporated by reference. Further, the viscoelasticreservoir element, functioning with a predetermined annular shearpreload provided via an adjustable preload flange, can dissipate anadditional portion of valve-closure impulse energy as heat while alsoattenuating and damping vibration. And viscoelastic fenestrationelements, when present, may contribute further to hysteresis loss asthey elastically retain the groove element in the seal-retention groovevia coupling to the reservoir element. Overall hysteresis loss in theviscoelastic elements combines with hysteresis loss in the valve body toselectively reduce the bandwidth, amplitude and duration of vibrationsthat the closing impulse energy would otherwise tend to excite in thevalve and/or pump housing.

Examples of mechanisms for such selective vibration reductions are seenin the interactions of the viscoelastic reservoir element with theadjustable preload flange. The interactions contribute to hysteresisloss in a tunable check valve assembly by, for example, creating whathas been termed shear damping (see, e.g., U.S. Pat. No. 5,670,006,incorporated by reference). With the preload flange adjustably fixedcentrally to the check valve body (e.g., fixed to a central guide stem),valve-closure impact causes both the preload flange and guide stem totemporarily move distally with respect to the (peripheral) valve seatinterface (i.e., the valve body experiences a concave-shaped flexure).The impact energy associated with valve closure causes temporarydeformation of the check valve body; that is, the valve body periphery(e.g., the valve seat interface) is stopped by contact with a valve seatwhile the central portion of the valve body continues (under inertialforces and pumped-fluid pressure) to elastically move distally. Thus,the annular constrained area of the viscoelastic reservoir element(shown constrained by the preload flange in the schematic illustrationsherein) moves substantially countercurrent (i.e., in shear) relative tothe annular unconstrained area (shown radially farther from the guidestem and peripheral to the preload flange). That is, relative distalmovement of the preload flange thus tends to extrude the (moreperipheral) annular unconstrained area proximally. Energy lost (i.e.,dissipated) in connection with the resulting shear strain in theviscoelastic element is subtracted from the total closing impulse energyotherwise available to excite destructive flow-induced vibrationresonances in a valve, valve seat and/or pump housing. See, e.g., U.S.Pat. No. 5,158,162, incorporated by reference.

Note that in viscoelastic and shear-thickening materials, therelationship between stress and strain (and thus the effect of materialconstraint on resonant frequency) is generally time-dependent andnon-linear. So a desired degree of non-linearity in “tuning” may bepredetermined by appropriate choice of viscoelastic and shear-thickeningmaterials in a tunable check valve assembly or tunable check valve.

Another aspect of the interaction of the viscoelastic reservoir elementwith an adjustable preload flange contributes to vibration dampingand/or absorption in a tunable check valve assembly. As a result ofcompliance in the viscoelastic element, longitudinal movement of a guidestem and a coupled preload flange results in a phase lag as shear stressdevelops within the viscoelastic material. This is analogous to thephase lag seen in the outer ring movement in an automotive torsionalvibration damper or the antiphase movement of small masses in anautomotive pendulum vibration damper. See, e.g., the '776 patent citedabove. Adjusting the shear preload flange as described above effectivelychanges the tunable check valve assembly's compliance and thus thedegree of phase lag. One may thus, in one or more limited operationalranges, tune viscoelastic element preload to achieve effective vibrationdamping plus dynamic vibration absorption at specific frequencies ofinterest (e.g., pump housing resonant frequencies).

To achieve the desired hysteresis loss associated with attenuation andvibration damping effects described herein, different viscoelasticand/or composite elements may be constructed to have specific elasticand/or viscoelastic properties. Note that the term elastic hereinimplies substantial characterization by a storage modulus, whereas theterm viscoelastic herein implies substantial characterization by astorage modulus and a loss modulus. See, e.g., the '006 patent citedabove.

Specific desired properties for each viscoelastic element arise from adesign concept requiring coordinated functions depending on the locationof each element. The viscoelastic reservoir element affects hysteresisassociated with longitudinal compliance of the tunable check valveassembly because it viscoelasticly accommodates longitudinal deformationof the valve body toward a concave shape. Hysteresis in the viscoelasticgroove element (related, e.g., to its valve seal and vibration dampingfunctions) and the valve body itself further reduces closing energyimpulse amplitude through dissipation of portions of closing impulseenergy as heat.

Elastic longitudinal compliance of a tunable check valve assemblyresults in part from elastic properties of the materials comprising thetunable check valve assembly. Such elastic properties may be achievedthrough use of composites comprising reinforcement materials as, forexample, in an elastic valve body comprising steel, carbon fiberreinforced polymer, carbon nanotube/graphene reinforced polymer, and/orcarbon nanotube/graphene reinforced metal matrix. The polymer maycomprise a polyaryletherketone (PAEK), for example, polyetheretherketone(PEEK). See, e.g., U.S. Pat. No. 7,847,057 B2, incorporated byreference.

Note that the description herein of valve body flexure as concave-shapedrefers to a view from the proximal or high-pressure side of the valvebody. Such flexure is substantially elastic and may be associated withslight circular rotation (i.e., a circular rolling contact) of the valvebody's valve seat interface with the valve seat itself. When the degreeof rolling contact is sufficient to justify conversion of the valve seatinterface from a conventional frusto-conical shape to a convex curvedshape (which may include, e.g., circular, elliptic and/or parabolicportions), a curved concave tunable valve seat mating surface may beused. In such cases, the valve seat interface has correspondinglygreater curvature than the concave tunable valve seat mating surface(see Detailed Description herein). Such rolling contact, when present,augments elastic formation of the concave valve body flexure on the pumppressure stroke, reversing the process on the suction stroke.

The circular rolling contact described herein may be visualized byconsidering the behavior of the convex valve seat interface as the valvebody experiences concave flexure (i.e., the transformation from arelatively flat shape to a concave shape). During such flexure theperiphery of the valve seat interface rotates slightly inwardly andtranslates slightly proximally (relative to the valve body's center ofgravity) to become the proximal rim of the concave-shaped flexure.

While substantially elastic, each such valve body flexure is associatedwith energy loss from the closing energy impulse due to hysteresis inthe valve body. Frictional heat loss (and any wear secondary tofriction) associated with any circular rolling contact of the convexvalve seat interface with the concave tunable valve seat mating surfaceis intentionally relatively low. Thus, the rolling action, when present,minimizes wear that might otherwise be associated with substantiallysliding contact of these surfaces. Further, when rolling contact betweenvalve body and tunable valve seat is present during both longitudinalvalve body flexure and the elastic rebound which follows, trapping ofparticulate matter from the pumped fluid between the rolling surfacestends to be minimized.

Since rolling contact takes place over a finite time interval, it alsoassists in smoothly redirecting pumped fluid momentum laterally andproximally. Forces due to oppositely directed radial components of theresultant fluid flow tend to cancel, and energy lost in pumped fluidturbulence is subtracted (as heat) from that of the valve-closure energyimpulse, thus decreasing both its amplitude and the amplitude ofassociated vibration.

In addition to the above described energy dissipation (associated withhysteresis secondary to valve body flexure), hysteresis loss will alsooccur during pressure-induced movements of the viscoelastic grooveelement (in association with the valve seal function). Note that pumpedfluid pressure acting on a valve comprising an embodiment of theinvention's tunable check valve assembly may hydraulically pressurizesubstantially all of the viscoelastic elements in a tunable check valveassembly. Although polymers suitable for use in the viscoelasticelements generally are relatively stiff at room ambient pressures andtemperatures, the higher pressures and temperatures experienced duringpump pressure strokes tend to cause even relatively stiff polymers tobehave like fluids which can transmit pressure hydraulically. Thus, aviscoelastic element in a peripheral seal-retention groove isperiodically hydraulically pressurized, thereby increasing its sealingfunction during the high-pressure portion of the pump cycle. Hydraulicpressurization of the same viscoelastic element is reduced during thelow-pressure portion of the pump cycle when the sealing function is notneeded.

Because of the above-described energy loss and the time required forvalve body longitudinal deformation to take place, with the associateddissipation of closing impulse energy described above, a valve-closureenergy impulse applied to a tunable check valve assembly or tunableradial array is relatively lower in amplitude and longer in duration(e.g., secondary to having a longer rise time) than an analogousvalve-closure energy impulse applied to a conventionally stiff valvebody which closes on a conventional frusto-conical valve seat. Thecombination of lower amplitude and increased duration of thevalve-closure energy impulse results in a narrowed characteristicvibration bandwidth having reduced potential for induction of damagingresonances in the valve, valve seat, and adjacent portions of the pumphousing. See, e.g., the above-cited '242 patent.

Note that in describing the fluid-like behavior of certain polymersherein under elevated heat and pressure, the term “polymer” includesrelatively homogenous materials (e.g., a single-species fluid polymer)as well as composites and combination materials containing one or moreof such relatively homogenous materials plus finely divided particulatematter (e.g., nanoparticles) and/or other dispersed species (e.g.,species in colloidal suspension, graphene) to improve heat scavengingand/or other properties. See, e.g., U.S. Pat. No. 6,432,320 B1,incorporated by reference.

In addition to heat scavenging, damping is a function of theviscoelastic elements in various embodiments of the invention. Optimaldamping is associated with relatively high storage modulus and losstangent values, and is obtained over various temperature ranges inmulticomponent systems described as having macroscopicallyphase-separated morphology, microheterogeneous morphology, and/or atleast one interpenetrating polymer network. See, e.g., the above-cited'006 patent and U.S. Pat. Nos. 5,091,455; 5,238,744; 6,331,578 B1; and7,429,220 B2, all incorporated by reference.

Summarizing salient points of the above description, recall thatvibration attenuation and damping in a tunable check valve assembly,tunable valve seat, tunable plunger seal, or tunable radial array of theinvention operate via four interacting mechanisms. First, impulseamplitude is reduced by converting a portion of total closing impulseenergy to heat (e.g., via hysteresis and fluid turbulence), which isthen ultimately rejected to the check valve body surroundings (e.g., thepumped fluid). Each such reduction of impulse amplitude means loweramplitudes in the characteristic vibration spectrum transmitted to thepump housing.

Second, the closing energy impulse as sensed at the valve seat isreshaped in part by lengthening the rebound cycle time (estimated as thetotal time associated with peripheral valve seal compression, concavevalve body flexure and elastic rebound). Such reshaping may in generalbe accomplished using mechanical/hydraulic/pneumatic analogs ofelectronic wave-shaping techniques. In particular, lengthened reboundcycle time is substantially influenced by the valve body's increasedlongitudinal compliance associated with the rolling contact/seal andconcave valve body flexure described herein between valve body and valveseat. The units of lengthened cycle times are seconds, so their inversefunctions have dimensions of per second (or 1/sec), the same dimensionsas frequency. Thus, as noted above, the inverse function is termedherein characteristic rebound frequency.

Lowered characteristic rebound frequency (i.e., increased rebound cycletime) corresponds to slower rebound, with a corresponding reduction ofthe impulse's characteristic bandwidth due to loss of higher frequencycontent. This condition is created during impulse hammer testing byadding to hammer head inertia and by use of softer impact tips (e.g.,plastic tips instead of the metal tips used when higher frequencyexcitation is desired). In contrast, tunable check valve assemblies andtunable radial arrays achieve bandwidth narrowing (and thus reduction ofthe damage potential of induced higher-frequency vibrations) at least inpart through increased longitudinal compliance. In other words,bandwidth narrowing is achieved in embodiments of the invention throughan increase of the effective impulse duration (as by, e.g., slowing theimpulse's rise time and/or fall time as the valve assembly's componentsflex and relax over a finite time interval).

Third, induced vibration resonances of the tunable check valve assembly,tunable valve seat, and/or other tunable components are effectivelydamped by interactions generating structural hysteresis loss. Associatedfluid turbulence further assists in dissipating heat energy via thepumped fluid.

And fourth, the potential for excitation of damaging resonances in pumpvibration induced by a closing energy impulse is further reduced throughnarrowing of the impulse's characteristic vibration bandwidth byincreasing the check valve body's effective inertia without increasingits actual mass. Such an increase of effective inertia is possiblebecause a portion of pumped fluid moves with the valve body as it flexesand/or longitudinally compresses. The mass of this portion of pumpedfluid is effectively added to the valve body's mass during the period offlexure/rebound, thereby increasing the valve body's effective inertiato create a low-pass filter effect (i.e., tending to block higherfrequencies in the manner of an engine mount).

To increase understanding of the invention, certain aspects of tunablecomponents (e.g., alternate embodiments and multiple functions ofstructural features) are considered in greater detail. Alternateembodiments are available, for example, in guide means known to thoseskilled in the art for maintaining valve body alignment within a(suction or discharge) bore. Guide means thus include, e.g., a centralguide stem and/or a full-open or wing-guided design (i.e., having adistal crow-foot guide).

Similarly, alteration of a viscoelastic element's vibration pattern(s)in a tunable fluid end is addressed (i.e., tuned) via adjustable and/ortime-varying constraints. Magnitude and timing of the constraints aredetermined in part by closing-impulse-related distortions and/or theassociated vibration. For example, a viscoelastic reservoir (or central)element is at least partially constrained as it is disposed in thecentral annular reservoir, an unconstrained area optionally being opento pumped fluid pressure. That is, the viscoelastic reservoir element isat least partially constrained by relative movement of the interiorsurface(s) of the (optionally annular) reservoir, and furtherconstrained by one or more structures (e.g., flanges) coupled to suchsurface(s). Analogously, a viscoelastic groove (or peripheral) elementis at least partially constrained by relative movement of the groovewalls, and further constrained by shear-thickening material within oneor more circular tubular areas of the element (any of which may comprisea plurality of lumens).

Since the magnitude and timing of closing-impulse-related distortionsare directly related to each closing energy impulse, the tunable fluidend's overall response is adaptive to changing pump operating pressuresand speeds on a stroke-by-stroke basis. So for each set of operatingparameters (e.g. cycle time and peak pressure for each pressure/suctionstroke cycle), one or more of the constrained viscoelastic elements hasat least a first predetermined assembly resonant frequency substantiallysimilar to an instantaneous pump resonant frequency (e.g., a resonantfrequency measured or estimated proximate the suction valve seat deck).And for optimal damping, one or more of the constrained viscoelasticelements may have, for example, at least a second predetermined assemblyresonant frequency similar to the first predetermined assembly resonantfrequency.

Note that the adaptive behavior of viscoelastic elements is beneficiallydesigned to complement both the time-varying behavior of valvesgenerating vibration with each pump pressure stroke, and thetime-varying response of the fluid end as a whole to that vibration.

Note also that a tunable check valve assembly and/or tunable valve seatanalogous to those designed for use in a tunable suction check valve maybe incorporated in a tunable discharge check valve as well. Either atunable suction check valve or a tunable discharge check valve or bothmay be installed in a pump fluid end housing. Additionally, one or moreother tunable components may be combined with tunable suction and/ordischarge check valves. A pump housing resonant frequency may be chosenas substantially equal to a first predetermined resonant frequency ofeach of the tunable components installed, or of any combination of theinstalled tunable components. Or the predetermined component resonantfrequencies may be tuned to approximate different pump housing resonantfrequencies as determined for optimal vibration damping.

For increased flexibility in accomplishing the above tuning,fenestrations may be present in the groove wall to accommodate radialtension members. At least a portion of each fenestration may have atransverse area which increases with decreasing radial distance to saidlongitudinal axis. That is, each fenestration flares to greatertransverse areas in portions closer to the longitudinal axis, relativeto the transverse areas of portions of the fenestration which are moredistant from the longitudinal axis. Thus, a flared fenestration ispartly analogous to a conventionally flared tube, with possibledifferences arising from the facts that (1) fenestrations are notlimited to circular cross-sections, and (2) the degree of flare maydiffer in different portions of a fenestration. Such flares assist instabilizing a viscoelastic groove element via a plurality of radialtension members.

Note that in addition to the example alternate embodiments describedherein, still other alternative invention embodiments exist, includingvalves, pump housings and pumps comprising one or more of the exampleembodiments or equivalents thereof. Additionally, use of a variety offabrication techniques known to those skilled in the art may lead toembodiments differing in detail from those schematically illustratedherein. For example, internal valve body spaces may be formed duringfabrication by welding (e.g., inertial welding or laser welding) valvebody portions together as in the above-cited '837 patent, or byseparately machining such spaces with separate coverings. Valve bodyfabrication may also be by rapid-prototyping (i.e., layer-wise)techniques. See, e.g., the above-cited '057 patent. Viscoelasticelements may be cast and cured separately or in place in a valve body asdescribed herein. See, e.g., U.S. Pat. No. 7,513,483 B1, incorporated byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic 3-dimensional view of a partially sectionedtunable check valve assembly/tunable radial array embodiment showing howan adjustable preload flange constrains an area of the viscoelasticreservoir element as described herein.

FIG. 2 includes a schematic 3-dimensional exploded view of the tunablecheck valve assembly/tunable radial array embodiment of FIG. 1 showingviscoelastic body elements, the valve body, and the adjustable preloadflange.

FIG. 3 is a schematic 3-dimensional partially-sectioned view ofviscoelastic reservoir, groove and fenestration elements (i.e.,viscoelastic body elements) of FIGS. 1 and 2 showing the constrainedarea of the reservoir element where it contacts an adjustable preloadflange, as well as an adjacent unconstrained area.

FIG. 4 is a schematic 3-dimensional partially-sectioned view of twocheck valve bodies with an adjustable preload flange located atdifferent longitudinal positions on a central guide stem.

FIG. 5 is a schematic 3-dimensional instantaneous partially-sectionedview of shear-thickening material which would, e.g., substantially filla circular tubular area in a viscoelastic groove element, a supportcircular tubular area of a tunable valve seat, a tunable plunger seal,or a tunable resilient circumferential seal.

FIG. 6 is a schematic illustration of an exploded partially-sectioned2-dimensional view of major components of a pump fluid end subassembly,together with brief explanatory comments on component functions. Theschematically-illustrated subassembly comprises a pumping chamber withina subassembly pump housing, the pumping chamber being in fluidcommunication with a suction bore, a discharge bore, and apiston/plunger bore. Schematic representations of a suction check valve,a discharge check valve, and a piston/plunger are shown in theirrespective bores, together with brief annotations and graphical aidsoutlining the structural relationships.

FIG. 7 is a schematic illustration of two views of an explodedpartially-sectioned 3-dimensional view of a valve body and tunable valveseat embodiment. Curved longitudinal section edges of the valve body'sconvex valve seat interface and corresponding concave mating portions ofthe tunable valve seat are shown schematically in a detail breakout viewto aid description herein of a rolling valve seal along a circular line.A tunable (suction or discharge) check valve embodiment of the inventionmay comprise a combination of a tunable check valve assembly/tunableradial array (see, e.g., FIGS. 1 and 2) and a tunable valve seat (see,e.g., FIGS. 7 and 8).

FIG. 8 is a schematic 3-dimensional exploded and partially-sectionedview of a tunable valve seat embodiment showing a concave mating surfacelongitudinally spaced apart from a lateral support mounting surface, andan adjustable lateral support assembly comprising first and secondsecurable end spacers in combination with a plurality of circularviscoelastic support elements, each support element comprising a supportcircular tubular area.

FIG. 9 is a schematic 3-dimensional exploded view of a partiallysectioned tunable check valve assembly embodiment. A dilatant (i.e.,shear-thickening) liquid is schematically shown being added to a checkvalve body's internal cavity, the cavity being shown as enclosing atuned vibration damper comprising discrete mechanical elements (e.g., amass and three springs).

FIG. 10 is a schematic 3-dimensional exploded view of a tunable checkvalve embodiment comprising the tunable check valve assembly of FIG. 9together with a tunable valve seat, the tunable check valve embodimentincluding structures to facilitate a rolling seal along a circular linebetween the valve body's valve seat interface and the tunable valveseat's mating surface. Note that the (convex) valve seat interface hascorrespondingly greater curvature than the (concave) mating surface, andthe mating surface has correspondingly less curvature than the valveseat interface.

FIG. 11 is a schematic 3-dimensional exploded view of an alternatetunable check valve embodiment comprising the tunable check valveassembly of FIG. 9 together with a tunable valve seat, the tunable checkvalve embodiment including structures to facilitate a rolling seal alonga circular line between the check valve body's peripheral valve seatinterface and the tunable valve seat's mating surface. An adjustablelateral support assembly is shown with the tunable valve seat, theassembly comprising first and second securable end spacers incombination with a plurality of circular viscoelastic support elements,each support element shown in a detail breakout view as comprising asupport circular tubular area.

FIG. 12 illustrates two schematic 3-dimensional views of an alternatetunable check valve assembly embodiment comprising a plurality ofradially-spaced vibration dampers disposed in a valve body having aperipheral seal. Each vibration damper comprises a circular tubulararea, and at least one vibration damper is tunable via a fluid medium(shown schematically being added) in a tubular area.

FIG. 13 is a schematic 3-dimensional exploded view of the alternatetunable check valve assembly embodiment of FIG. 12. Detail breakoutviews include the peripheral seal's medial flange and the flange body'scorresponding flange channel. An instantaneous schematic view of thefluid medium in the peripheral seal's circular tubular area is shownseparately.

FIG. 14 illustrates a partial schematic 3-dimensional view of analternate tunable check valve embodiment comprising the valve body ofFIGS. 12 and 13, together with a tunable valve seat. A detail breakoutview shows that the valve seat interface has correspondingly greatercurvature than the mating surface. The mating surface hascorrespondingly less curvature than the valve seat interface tofacilitate a rolling seal along a circular line between the valve body'svalve seat interface and the tunable valve seat's mating surface.

FIG. 15 illustrates a partial schematic 3-dimensional view of a tunablehydraulic stimulator embodiment comprising a driver element and a hammerelement in a hollow cylindrical housing, one end of the housing beingclosed by a fluid interface, and the fluid interface comprising a MEMSaccelerometer.

FIG. 16 illustrates a partial schematic 3-dimensional exploded view ofthe tunable hydraulic stimulator embodiment of FIG. 15, a firstelectrical cable being shown to schematically indicate a feedback path(for an accelerometer signal) from the accelerometer to the driverelement. A second electrical cable is shown to schematically indicate aninterconnection path for, e.g., communication with one or moreadditional stimulators and/or associated equipment.

FIG. 17 schematically illustrates a 2-dimensional view of majorcomponents, subsystems, and interconnections of a tunable down-holestimulation system, together with brief explanatory comments oncomponent and subsystem functions. As aids to orientation, a schematicwellbore is shown, as are control link pathways for communication amongpumps, tunable down-hole stimulator(s) and a tunable down-holestimulation system controller. Schematic pathways are shown forstimulation vibration energy directed toward down-hole geologic materialadjacent to the wellbore, and for backscatter vibration energy emanatingfrom the geologic material.

DETAILED DESCRIPTION

Tunable equipment associated with high-pressure well-stimulationcomprises a first family of tunable down-hole stimulators (plusassociated controllers, power supplies, etc.), a second family of (frac)pumps optionally comprising tunable fluid ends (which include but arenot limited to, e.g., tunable valve assemblies and/or vibrationdampers), and a third family of closed-loop tunable down-holestimulation systems. An example tunable down-hole stimulation systemcomprises at least one frac pump, at least one tunable down-holestimulator, and at least one tunable down-hole stimulation systemprogrammable controller. The controller has control links to at leastone frac pump and at least one tunable down-hole stimulator. Note thateach frac pump in a tunable down-hole stimulation system may optionallycomprise one or more components of a tunable fluid end.

The above three equipment families have strikingly different operationalcharacteristics. In the first family, tunable down-hole stimulatorsoperate within the wellbore, generating and transmitting vibrationtailored to enhance stimulation of geologic materials for higherhydrocarbon yields. In contrast, fluid ends of the second family mayhave one or more installed tunable components which facilitate selectiveattenuation of valve-generated vibration at or near its source to reducefluid end fatigue failures. The third family of closed-loop tunabledown-hole stimulation systems includes system controllers pluselaborations of the first family's tunable hydraulic stimulators, withoptional inclusion of the second family's tunable fluid ends. Structuresrelated to the first and second families are shown in FIGS. 1-16, whileFIG. 17 schematically illustrates various structures and functions ofthird-family systems.

FIGS. 1-14 relate generally to the above second tunable equipmentfamily. They schematically illustrate how adding tunable valve seats,tunable radial arrays and/or plunger seals to tunable check valveassemblies in a fluid end further facilitates optimal damping and/orselective attenuation of vibration at one or more predetermined (andfrequently-localized) fluid end resonant frequencies. Optimizedvibration attenuation (via, e.g., optimized fluid end damping) isprovided by altering resonant frequencies in each tunable component inrelation to one or more (measured or estimated) fluid end resonantfrequencies and/or tunable component resonant frequencies.

A tunable (suction or discharge) check valve of the invention maycomprise, for example, a combination of a tunable check valveassembly/tunable radial array 99 (see, e.g., FIG. 1) and a tunable valveseat 20 or a tunable valve seat 389 (see, e.g., FIGS. 7 and 11). Detailsof the structure and functions of each component are provided hereinboth separately and as combined with other components to obtainsynergistic benefits contributing to longer pump service life.

FIGS. 1 and 2 schematically illustrate an invention embodiment of atunable check valve assembly/tunable radial array 99 substantiallysymmetrical about a longitudinal axis. Illustrated components include avalve body 10, an adjustable preload flange 30, and a plurality ofviscoelastic body elements 50. Check valve body 10, in turn, comprises aperipheral groove 12 (see FIG. 2) spaced apart by an annular (central)reservoir 16 from a longitudinal guide stem 14, groove 12 beingresponsive to longitudinal compressive force. A plurality ofviscoelastic body elements 50 comprises an annular (central) reservoirelement 52 coupled to a (peripheral) groove element 54 by a plurality of(optional) radial fenestration elements 56 (in fenestrations 18) to forma tunable radial array. Groove element 54 functions as a vibrationdamper and valve seal, comprising at least one circular tubular area 58.

Responsiveness of groove 12 to longitudinal compressive force ischaracterized in part by damping of groove wall 11/13/15 vibrations.Such damping is due in part to out-of-phase vibrations in proximalgroove wall 13 and distal groove wall 11 which are induced bylongitudinal compressive force. Such out-of-phase vibrations will causevarious groove-related dimensions to vary with longitudinal compressiveforce, thereby indicating the responsiveness of groove 12 to such force(see, for example, the dimension labeled A in FIG. 2). Each phase shift,in turn, is associated with differences in the coupling of proximalgroove wall 13 to guide stem 14 (indirectly via longitudinal groove wall15 and radial reservoir floor 19) and the coupling of distal groove wall11 to guide stem 14 (directly via radial reservoir floor 19). Note thatlongitudinal groove wall 15 may comprise fenestrations 18, therebyincreasing the responsiveness of groove 12 to longitudinal compressiveforce on tunable check valve assembly 99.

Referring to FIGS. 1-3, adjustable preload flange 30 extends radiallyfrom guide stem 14 (toward peripheral reservoir wall 17) over, forexample, about 20% to about 80% of viscoelastic reservoir element 52(see FIG. 3). Adjustable preload flange 30 thus imposes an adjustableannular shear preload over an annular constrained area 62 ofviscoelastic reservoir element 52 to achieve at least a firstpredetermined assembly resonant frequency substantially replicating a(similar) measured or estimated resonant frequency (e.g., a pump housingresonant frequency). Note that an adjacent annular unconstrained area 60of viscoelastic reservoir element 52 remains open to pumped fluidpressure. Note also that adjustable preload flange 30 may be adjusted ineffective radial extent and/or longitudinal position.

Note further that annular constrained area 62 and annular unconstrainedarea 60 are substantially concentric and adjacent. Thus, for a tunablesuction valve subject to longitudinal (i.e., distally-directed)compressive constraint applied via preload flange 30 to annularconstrained area 62, annular unconstrained area 60 will tend to move(i.e., extrude) proximally relative to area 62. The oppositely-directed(i.e., countercurrent) movements of constrained and unconstrainedannular areas of viscoelastic reservoir element 52 create asubstantially annular area of shear stress.

Finally, each circular tubular area 58 is substantially filled with atleast one shear-thickening material 80 (see FIG. 5) chosen to achieve atleast a second predetermined assembly resonant frequency similar, forexample, to the first predetermined assembly resonant frequency). Notethat FIG. 5 schematically represents a partially-sectioned view of aninstantaneous configuration of the shear-thickening material 80 withincircular tubular area 58.

Referring to FIGS. 1 and 2 in greater detail, a tunable check valveassembly/tunable radial array embodiment 99 comprises viscoelastic bodyelements 50 which comprise, in turn, reservoir (central) element 52coupled to groove (peripheral) element 54 via radial fenestration(tension) elements 56. Elements 52, 54 and 56 are disposed in (i.e.,integrated with and/or lie substantially in) reservoir 16, groove 12 andfenestrations 18 respectively to provide a tuned radial array having atleast a third predetermined resonant frequency. An adjustable preloadflange 30 is coupled to guide stem 14 and contacts viscoelasticreservoir element 52 in reservoir 16 to impose an adjustable annularconstraint on viscoelastic reservoir element 52 for achieving at least afirst predetermined assembly resonant frequency substantially similarto, for example, a measured resonant frequency (e.g., a pump housingresonant frequency). Such adjustable annular constraint imposes anadjustable shear preload between constrained annular area 62 andunconstrained annular area 60. Tunable check valve assembly 99 mayadditionally comprise at least one circular tubular area 58 in grooveelement 54 residing in groove 12, each tubular area 58 beingsubstantially filled with at least one shear-thickening material 80chosen to achieve at least a second predetermined assembly resonantfrequency similar, for example, to the first predetermined assemblyresonant frequency).

The above embodiment may be installed in a pump housing having ameasured housing resonant frequency; the measured housing resonantfrequency may then be substantially replicated in the (similar) firstpredetermined resonant frequency of the tunable check valve assembly.Such a combination would be an application of an alternate embodiment.An analogous tuning procedure may be followed if the tunable check valveassembly of the second embodiment is installed in a pump having a(similar or different) resonant frequency substantially equal to thesecond predetermined resonant frequency. This synergistic combinationwould broaden the scope of the valve assembly's beneficial effects,being yet another application of the invention's alternate embodiment.

Note that preload flange 30 may have a non-cylindrical periphery 32 forimposing on viscoelastic reservoir element 52 an adjustable annularshear preload having both longitudinal and transverse components.

Note further that the periphery of adjustable preload flange 30, ifcylindrical, predisposes a tunable check valve assembly to substantiallylongitudinal shear damping with each longitudinal distortion of checkvalve body 10 associated with valve closure. The character of such sheardamping depends, in part, on the longitudinal position of the preloadflange. Examples of different longitudinal positions are seen in FIG. 4,which schematically illustrates the flange 30′ longitudinally displacedfrom flange 30″. Further, as shown in FIG. 4, the convex periphery of alongitudinally adjusted preload flange 30′ or 30″ may introduce sheardamping of variable magnitude and having both longitudinal andtransverse components. Such damping may be beneficial in cases wheresignificant transverse valve-generated vibration occurs.

To clarify the placement of viscoelastic body elements 50, labelsindicating the portions are placed on a sectional view in FIGS. 2 and 3.Actual placement of viscoelastic body elements 50 in valve body 10 (seeFIG. 1) may be by, for example, casting viscoelastic body elements 50 inplace, or placing viscoelastic body elements 50 (which have beenprecast) in place during layer-built or welded fabrication. The tunablecheck valve assembly embodiment of the invention is intended torepresent check valve body 10 and viscoelastic body elements 50 ascomplementary components at any stage of manufacture leading tofunctional integration of the two components.

To enhance scavenging of heat due to friction loss and/or hysteresisloss, shear-thickening material 80 and/or viscoelastic body elements 50may comprise one or more polymers which have been augmented withnanoparticles and/or graphene 82 (see, e.g., FIG. 5). Nanoparticlesand/or graphene may be invisible to the eye as they are typicallydispersed in a colloidal suspension. Hence, they are schematicallyrepresented by cross-hatching 82 in FIG. 5. Nanoparticles may comprise,for example, carbon forms (e.g., graphene) and/or metallic materialssuch as copper, beryllium, titanium, nickel, iron, alloys or blendsthereof. The term nanoparticle may conveniently be defined as includingparticles having an average size of up to about 2000 nm. See, e.g., the'320 patent.

FIG. 6 is a schematic illustration of an exploded partially-sectioned2-dimensional view of major components of a pump fluid end subassembly88, together with graphical aids and brief explanatory comments oncomponent functions. The schematically-illustrated subassembly 88comprises a pumping chamber 74 within a subassembly (pump) housing 78,the pumping chamber 74 being in fluid communication with a suction bore76, a discharge bore 72, and a piston/plunger bore 70. Note thatpiston/plunger bore 70 comprises at least one recess (analogous to thatlabeled “packing box” in FIG. 6) in which at least one lateral supportassembly 130 (see FIG. 8) may be sealingly positionable along theplunger as part of a tunable plunger seal embodiment. Schematicrepresentations of a tunable suction valve 95 (illustrated forsimplicity as a hinged check valve), a tunable discharge valve 97 (alsoillustrated for simplicity as a hinged check valve), and apiston/plunger 93 (illustrated for simplicity as a plunger) are shown intheir respective bores. Note that longitudinally-moving valve bodies incheck valve embodiments schematically illustrated herein (e.g., valvebody 10) are associated with certain operational phenomena analogous tophenomena seen in hinged check valves (including, e.g., structuralcompliance secondary to closing energy impulses).

Regarding the graphical aids of FIG. 6, the double-ended arrows thatsignify fluid communication between the bores (suction, discharge andpiston/plunger) and the pumping chamber are double-ended to representthe fluid flow reversals that occur in each bore during each transitionbetween pressure stroke and suction stroke of the piston/plunger. Thelarge single-ended arrow within the pumping chamber is intended torepresent the periodic and relatively large, substantiallyunidirectional fluid flow from suction bore through discharge boreduring pump operation.

Further regarding the graphical aids of FIG. 6, tunable suction (check)valve 95 and tunable discharge (check) valve 97 are shown schematicallyas hinged check valves in FIG. 6 because of the relative complexity ofcheck valve embodiments having longitudinally-moving valve bodies. Moredetailed schematics of several check valve assemblies and elements areshown in FIGS. 1-11, certain tunable check valve embodiments comprisinga tunable check valve assembly and a tunable valve seat. In general, thetunable check valve assemblies/tunable radial arrays of tunable suctionand discharge valves will typically be tuned to different assemblyresonant frequencies because of their different positions in asubassembly housing 78 (and thus in a pump housing as described herein).Pump housing resonant frequencies that are measured proximate thetunable suction and discharge valves will differ in general, dependingon the overall pump housing design. In each case they serve to guide thechoices of the respective assembly resonant frequencies for the valves.

Note that the combination of major components labeled in FIG. 6 as apump fluid end subassembly 88 is so labeled (i.e., is labeled as asubassembly) because typical fluid end configurations comprise aplurality of such subassemblies combined in a single machined block.Thus, in such typical (multi-subassembly) pump fluid end designs, aswell as in less-common single-subassembly pump fluid end configurations,the housing is simply termed a “pump housing” rather than the“subassembly housing 78” terminology of FIG. 6.

Further as schematically-illustrated and described herein for clarity,each pump fluid end subassembly 88 comprises only major components: apumping chamber 74, with its associated tunable suction valve 95,tunable discharge valve 97, and piston/plunger 93 in their respectivebores 76, 72 and 70 of subassembly housing 78. For greater clarity ofdescription, common fluid end features well-known to those skilled inthe art (such as access bores, plugs, seals, and miscellaneous fixtures)are not shown. Similarly, a common suction manifold through whichincoming pumped fluid is distributed to each suction bore 76, and acommon discharge manifold for collecting and combining discharged pumpedfluid from each discharge bore 72, are also well-known to those skilledin the art and thus are not shown.

Note that the desired check-valve function of tunable check valves 95and 97 schematically-illustrated in FIG. 6 requires interaction of therespective tunable check valve assemblies (see, e.g., FIGS. 1-5) with acorresponding (schematically-illustrated) tunable valve seat (see, e.g.,FIGS. 7, 8, 10 and 11). The schematic illustrations of FIG. 6 are onlyintended to convey general ideas of relationships and functions of themajor components of a pump fluid end subassembly. Structural details ofthe tunable check valve assemblies that are in turn part of tunablecheck valves 95 and 97 of the invention (including their respectivetunable valve seats) are illustrated in greater detail in other figuresas noted above. Such structural details facilitate a plurality ofcomplementary functions that are best understood through reference toFIGS. 1-5 and 7-11.

The above complementary functions of tunable check valves include, butare not limited to, closing energy conversion to heat via structuralcompliance, energy redistribution through rejection of heat to thepumped fluid and pump housing, vibration damping and/or selectivevibration spectrum narrowing through changes in tunable check valveassembly compliance, vibration frequency down-shifting (via decrease incharacteristic rebound frequency) through increase of rebound cycletime, and selective vibration attenuation through energy dissipation(i.e., via redistribution) at predetermined assembly resonantfrequencies.

FIG. 7 is a schematic illustration of two views of an explodedpartially-sectioned 3-dimensional view including a check valve body 10and its convex valve seat interface 22, together with concave matingsurface 24 of tunable valve seat 20. Mating surface 24 is longitudinallyspaced apart from a pump housing interface surface 21. A curvedlongitudinal section edge 28 of the tunable valve seat's mating surface24, together with a correspondingly greater curved longitudinal sectionedge 26 of the valve body's valve seat interface 22, are shownschematically in detail view A to aid description herein of a rollingvalve seal.

The correspondingly greater curvature of valve seat interface 22, ascompared to the curvature of mating surface 24, effectively provides arolling seal against fluid leakage which reduces wear on the surfaces incontact. The rolling seal also increases longitudinal compliance of atunable suction or discharge valve of the invention, with the addedbenefit of increasing the rise and fall times of the closing energyimpulse (thus narrowing the associated vibration spectrum). Widening theclosing energy impulse increases rebound cycle time and correspondinglydecreases characteristic rebound frequency.

Further regarding the terms “correspondingly greater curvature” or“correspondingly less curvature” as used herein, note that thecurvatures of the schematically illustrated longitudinal section edges(i.e., 26 and 28) and the surfaces of which they are a part (i.e., valveseat interface 22 and mating surface 24 respectively) are chosen so thatthe degree of longitudinal curvature of valve seat interface 22(including edge 26) exceeds that of (i.e., has correspondingly greatercurvature than) mating surface 24 (including edge 28) at any point ofrolling contact. In other words, mating surface 24 (including edge 28)has correspondingly less curvature than valve seat interface 22(including edge 26). Hence, rolling contact (i.e., a rolling valve seal)between valve seat interface 22 and mating surface 24 is along asubstantially circular line (i.e., mating surface 24 is a curved matingsurface for providing decreased contract area along the circular line).The plane of the circular line is generally transverse to the(substantially coaxial) longitudinal axes of valve body 10 and tunablevalve seat 20. And the decreased contract area along the circular lineis so described because it is small relative to the nominal contact areaotherwise provided by conventional (frusto-conical) valve seatinterfaces and valve seat mating surfaces.

Note that the nominal frusto-conical contact area mentioned above iscustomarily shown in engineering drawings as broad and smooth. But theactual contact area is subject to unpredictable variation in practicedue to uneven distortions (e.g., wrinkling) of the respectiveclosely-aligned frusto-conical surfaces under longitudinal forces thatmay exceed 250,000 pounds. An advantage of the rolling valve seal alonga substantially circular line as described herein is minimization of theunpredictable effects of such uneven distortions of valve seatinterfaces and their corresponding mating surfaces.

Note also that although valve seat interface 22 and mating surface 24(and other valve seat interface/mating surface combinations describedherein) are schematically illustrated as curved, either may befrusto-conical (at least in part) in certain tuned componentembodiments. Such frusto-conical embodiments may have lower fabricationcosts and may exhibit suboptimal distortion, down-shifting performanceand/or wear characteristics. They may be employed in relativelylower-pressure applications where other tunable componentcharacteristics provide sufficient operational advantages in vibrationcontrol.

The above discussion of rolling contact applies to the alternate tunablevalve seat 20′ of FIG. 8, as it does to the tunable valve seat 20 ofFIG. 7. FIG. 8 schematically illustrates a 3-dimensional exploded andpartially-sectioned view of a tunable valve seat showing a matingsurface (analogous to mating surface 24 of FIG. 7) longitudinally spacedapart from a lateral support mounting surface 21′. But the lateralsupport mounting surface 21′ in FIG. 8 differs from pump housinginterface surface 21 of FIG. 7 in that it facilitates adjustablysecuring a lateral support assembly 130 to alternate tunable valve seat20′. Lateral support assembly 130 comprises first and second securableend spacers (110 and 124 respectively) in combination with a pluralityof circular viscoelastic support elements (114, 118 and 122), eachsupport element comprising a support circular tubular area (see areas112, 116 and 120 respectively). Shear-thickening material in eachsupport circular tubular area 112, 116 and 120 is chosen so each lateralsupport assembly 130 has at least one predetermined resonant frequency.Lateral support assemblies thus configured may be part of each tunablevalve seat and each tunable plunger seal. When part of a tunable plungerseal, one or more lateral support assemblies 130 reside in at least onerecess analogous to the packing box schematically illustrated adjacentto piston/plunger 93 (i.e., as a portion of piston/plunger bore 70) inFIG. 6.

Note also that in general, a tunable (suction or discharge) check valveof the invention may comprise a combination of a tunable check valveassembly 99 (see, e.g., FIG. 1) and a tunable valve seat 20 (see, e.g.,FIG. 7) or a tunable valve seat 20′ (see, e.g., FIG. 8). Referring morespecifically to FIG. 6, tunable suction check valve 95 is distinguishedfrom tunable discharge check valve 97 by one or more factors, includingeach measured resonant frequency to which each tunable check valve istuned so as to optimize the overall effectiveness of valve-generatedvibration attenuation in the associated pump housing 78.

FIGS. 9-11 show schematic exploded views of a nonlinear spring-massdamper 227/228/229/230, which may be incorporated in a tunable checkvalve assembly embodiment 210. FIGS. 9-11 can each be understood asschematically illustrating a tunable check valve assembly with orwithout a peripheral groove viscoelastic element. That is, each figuremay also be understood to additionally comprise a viscoelastic grooveelement analogous to groove element 54 (see FIG. 2) residing in groove218′/218″ (see FIG. 9)—this groove element is not shown in explodedFIGS. 9-11 for clarity, but may be considered to comprise at least onecircular tubular area analogous to tubular area 58 in groove element 54(see FIG. 2), each tubular area 58 being substantially filled with atleast one shear-thickening material 80 chosen to achieve at least onepredetermined assembly resonant frequency.

Referring to FIG. 9, Belleville springs 227/228/229 are nonlinear, andthey couple mass 230 to the valve body base plate 216 and the proximalvalve body portion 214. Additionally, dilatant liquid 242 is optionallyadded (via sealable ports 222 and/or 220) to central internal cavity 224to immerse nonlinear spring-mass damper 227/228/229/230. The nonlinearbehavior of dilatant liquid 242 in shear (as, e.g., between Bellevillesprings 227 and 228) expands the range of tuning the nonlinearspring-mass damper 227/228/229/230 to a larger plurality ofpredetermined frequencies to reduce “ringing” of valve body 214/216 inresponse to a closing energy impulse.

To clarify the function of nonlinear spring-mass damper 227/228/229/230,mass 230 is shown perforated centrally to form a washer shape and thusprovide a passage for flow of dilatant liquid 242 during longitudinalmovement of mass 230. This passage is analogous to that provided by eachof the Belleville springs 227/228/229 by reason of their washer-likeshape.

FIG. 10 shows an exploded view of an alternate embodiment of a tunablecheck valve comprising the tunable check valve assembly 210 of FIG. 9,plus a tunable valve seat 250. FIGS. 10 and 11 schematically illustratetwo views of an exploded partially-sectioned 3-dimensional viewincluding a valve body 214/216 and its valve seat interface 234,together with mating surface 254 of tunable valve seats 250 and 250′.Mating surface 254 is longitudinally spaced apart from pump housinginterface surface 252 in FIG. 10, and from lateral support mountingsurface 252′ in FIG. 11. In FIG. 10, a curved longitudinal section edge256 of the tunable valve seat's mating surface 254, together with acorrespondingly greater curved longitudinal section edge 236 of valveseat interface 234, are shown schematically to aid description herein ofa rolling valve seal along a substantially circular line.

Note that valve body 214/216 may be fabricated by several methods,including that schematically illustrated in FIGS. 9-11. For example,circular boss 215 on proximal valve body portion 214 may be inertiawelded or otherwise joined to circular groove 217 on valve body baseplate 216. Such joining results in the creation of peripheralseal-retention groove 218′/218″ having proximal groove wall 218′ anddistal groove wall 218″.

To enhance scavenging of heat due to friction loss and/or hysteresisloss, liquid polymer(s) 242 may be augmented by adding nanoparticleswhich are generally invisible to the eye as they are typically dispersedin a colloidal suspension. Nanoparticles comprise, for example, carbonand/or metallic materials such as copper, beryllium, titanium, nickel,iron, alloys or blends thereof. The term nanoparticle may convenientlybe defined as including particles having an average size of up to about2000 nm. See, e.g., the '320 patent.

The correspondingly greater curvature of valve seat interface 234, ascompared to the curvature of mating surface 254, effectively provides arolling seal against fluid leakage which reduces frictional wear on thesurfaces in contact. The rolling seal also increases longitudinalcompliance of a tunable suction or discharge valve of the invention,with the added benefit of increasing the rise and fall times of theclosing energy impulse (thus narrowing the associated vibrationspectrum).

Further regarding the term “correspondingly greater curvature” as usedherein, note that the curvatures of the schematically illustratedlongitudinal section edges (i.e., 236 and 256) and the surfaces of whichthey are a part (i.e., valve seat interface 234 and mating surface 254respectively) are chosen so that the degree of longitudinal curvature ofvalve seat interface 234 (including edge 236) exceeds that of (i.e., hascorrespondingly greater curvature than) mating surface 254 (includingedge 256) at any point of rolling contact. Hence, rolling contactbetween valve seat interface 234 and mating surface 254 is always alonga substantially circular line that decreases contact area relative tothe (potentially variable) contact area of a (potentially distorted)conventional frusto-conical valve body/valve seat interface (seediscussion above). The plane of the circular line is generallytransverse to the (substantially coaxial) longitudinal axes of valvebody 214/216 and tunable valve seat 250. (See notes above refrusto-conical valve seat interface shapes and mating surfaces).

The above discussion of rolling contact applies to the alternate tunablevalve seat 250′ of FIG. 11, as it does to the tunable valve seat 250 ofFIG. 10. But the lateral support mounting surface 252′ in tunable checkvalve 399 of FIG. 11 differs from pump housing interface surface 252 ofFIG. 10 in that it facilitates adjustably securing a lateral supportassembly 330 to alternate tunable valve seat 250′ to form tunable valveseat 389. Lateral support assembly 330 comprises first and secondsecurable end spacers (310 and 324 respectively) in combination with aplurality of circular viscoelastic support elements (314, 318 and 322),each support element comprising a support circular tubular area (312,316 and 320 respectively).

Note that in general, a tunable (suction or discharge) check valve ofthe invention may comprise a combination of a tunable check valveassembly 210 (see, e.g., FIG. 9) and a tunable valve seat 250 (see,e.g., FIG. 10) or a tunable valve seat 250′ (see, e.g., FIG. 11).Referring more specifically to FIG. 6, tunable suction valve 95 isdistinguished from tunable discharge check valve 97 by one or morefactors, including each measured or estimated resonant frequency towhich each tunable check valve is tuned so as to optimize the overalleffectiveness of valve-generated vibration attenuation in the associatedpump housing 78.

FIG. 12 illustrates two schematic 3-dimensional views of an alternatetunable check valve assembly embodiment 410/470/480 (see exploded viewin FIG. 13) which is symmetrical about a longitudinal axis and comprisesa plurality of radially-spaced vibration dampers. One such damper is inthe peripheral seal 470 with its peripheral circular tubular area 472and enclosed fluid tuning medium 482, tubular area 472 being responsiveto longitudinal compression of the assembly. A second damper is in valvebody 410 with enclosed spaces 460/464 in fluid communication withcentral circular tubular area 462 via fluid flow restrictors 466/468 inthe presence of fluid tuning medium 442. Tubular area 462 and fluid flowrestrictors 466/468 are also responsive to longitudinal to compressionof the assembly, thereby prompting fluid flow through the flowrestrictors in association with valve closure shock and/or vibration.

Thus, each vibration damper comprises a circular tubular area (462/472),and at least one vibration damper is tunable to a predeterminedfrequency (e.g., a resonant frequency of a fluid end in which theassembly is installed). The tuning mechanisms may differ: e.g., via afluid medium 442 (shown schematically being added in FIG. 12 via asealable port 422 in valve body 410) in a tubular area 462 and/or via afluid medium 482 (shown as an instantaneous shape 480) within tubulararea 472. Control of variable fluid flow resistance and/or fluidstiffness (in the case of shear-thickening fluids) facilitatespredetermination of resonant frequency or frequencies in the central andperipheral dampers.

In either case, tuning is function of responsiveness of the respectivedampers to vibration secondary to valve closure impact (see abovediscussion of such impact and vibration). For example, longitudinalforce on the closed valve will tend to reduce the distance betweenopposing fluid flow restrictors 466/468, simultaneously prompting flowof fluid tuning medium 442 from circular tubular area 462 to areas 464and/or 460 (460 acting as a surge chamber). Flow resistance will be afunction of fluid flow restrictors 466/468 and the fluid viscosity. Notethat viscosity may vary with time in a shear-thickening liquid 442,thereby introducing nonlinearly for predictably altering centerfrequency and/or Q of the damper. Analogous predetermined viscosityvariation in fluid tuning medium 482 is available for predictablyaltering the center frequency and/or Q (i.e., altering the tuning) ofthe peripheral damper 470/472/482 as the seal 470 distorts under thelongitudinal load of valve closure.

Note that the peripheral seal vibration damper 470/472/482 comprises amedial flange 479 sized to closely fit within flange channel 419 ofvalve body 410, and medial flange 419 partially surrounds circulartubular area 472 within said seal 470. Those skilled in the art knowthat conventional peripheral seals tend to rotate within their retaininggroove. The illustrated seal embodiment herein shows that such rotationwill tend to be resisted by the combined action of medial flange 479 andflange channel 419. Further, the portion of circular tubular area 472partially surrounded by medial flange 419 will tend to stiffen medialflange 479 in a nonlinear manner when circular tubular area 472 containsa shear-thickening fluid tuning medium.

FIG. 14 illustrates a partial schematic 3-dimensional view of analternate tunable check valve embodiment comprising the valve body 410of FIG. 13, together with a tunable valve seat 452. A detail breakoutview shows that the valve seat interface has correspondingly greatercurvature than the mating surface to facilitate a rolling valve sealalong a substantially circular line, the seal having predeterminedrebound cycle time and characteristic rebound frequency as describedabove.

FIGS. 15 and 16 illustrate partial schematic 3-dimensional views of atunable hydraulic stimulator embodiment 599, FIG. 16 being an explodedview. Numerical labels may appear in only one view. A hollow cylindricalhousing 590 has a longitudinal axis, a first end 594, and a second end592. First end 594 is closed by fluid interface 520 for transmitting andreceiving vibration. Fluid interface 520 comprises at least oneaccelerometer 518 for producing an accelerometer electrical signal(i.e., an accelerometer-generated feedback signal) representingvibration transmitted and received via fluid interface 520.

Driver element 560 (comprising a field emission structure which itselfcomprises electromagnet/controller 564/562) reversibly seals second end592, and hammer (or movable mass) element 540 is longitudinally movablewithin housing 590 between driver element 560 and fluid interface 520.In some embodiments, hammer element 540 may itself be a field emissionstructure consisting of a permanent magnet (or consisting of a pluralityof permanent magnets). Polarity of any such permanent magnets is notspecified because it would be assigned in light of theelectromagnet/controller 564/562. Alternatively, hammer element 540 maybe analogous in part to the armature of a linear electric motor, as in arailgun. (See, e.g., the '205 and '877 patents noted above). Note thatthe above accelerometer-generated feedback electrical signal may beaugmented by, or replaced by, sensorless control means (e.g.,controlling operating parameters of electromagnet 564 such as magneticfield strength and polarity) in free piston embodiments of the tunablehydraulic stimulator. (See, e.g., U.S. Pat. No. 6,883,333 B2,incorporated by reference).

Thus, hammer element 540 is responsive to the magnetic field emitted bydriver element 560 for striking, and rebounding from, fluid interface520. The duration of each such striking and rebounding cycle (termedherein the “hammer rebound cycle time”) has the dimension of seconds.And the inverse of this duration has the dimension of frequency. Hence,the term herein “characteristic rebound frequency” is the inverse of ahammer rebound cycle time, and the hammer rebound cycle time itself isinversely proportional to the bandwidth of transmitted vibration spectraresulting from each hammer strike and rebound from fluid interface 520.

Fluid interface 520 transmits vibration spectra generated by hammerimpacts on fluid interface 520 as well as receiving backscattervibration from geologic formations excited by stimulator 599. Fluidinterface 520 comprises, for example, a MEMS accelerometer 518 forproducing an accelerometer signal representing vibration transmitted andreceived by fluid interface 520. (See MicroElectro-Mechanical Systems inHarris, pp. 10-26, 10-27).

Hammer element 540 comprises a striking face 542 (see FIG. 16) which hasa predetermined modulus of elasticity (e.g., that of mild steel, about29,000,000 psi) which can interact with the modulus of elasticity offluid interface 520 (again, e.g., that of mild steel). In a practicalexample, interaction of the two suggested moduli of elasticitypredetermines a relatively short rebound cycle time for hammer element540, which is associated with a corresponding relatively broad-spectrumof vibration to be transmitted by fluid interface 520. In other words,striking face 542 strikes fluid interface 520 and rebounds to produce arelatively short-duration, high-amplitude mechanical shock. (See, e.g.,Harris p. 10.31).

Both FIGS. 15 and 16 schematically illustrate a tunable resilientcircumferential seal 580 for sealing housing 590 within a wellbore, thuspartially isolating vibration transmitted by fluid interface 520 withinthe wellbore. Seal 580 comprises at least one circular tubular area 582which may contain at least one shear-thickening fluid 80 (see FIG. 5)which is useful in part for tuning purposes. And fluid 80 may comprisenanoparticles 82 for, e.g., facilitating heat scavenging.

FIG. 16 also schematically illustrates a first electrical cable 516 forcarrying accelerometer electrical signals (schematically representingvibration data transmitted by and/or received by fluid interface 520)from accelerometer 518 to driver element 560. A second electrical cable514 also connects to driver element 560 of each tunable hydraulicstimulator to schematically represent interconnection of two or moresuch stimulators (to form a tunable hydraulic stimulator array) and/orfor connecting one or more down-hole tunable hydraulic stimulators torelated equipment (e.g., a programmable controller as shown in FIG. 17)proximal in a wellbore and/or adjacent to the wellhead. Accelerometerelectrical signals provide feedback on transmitted vibration and also onreceived backscatter vibration to driver element 560.

While accelerometer-mediated feedback may be desired for tailoringstimulation to specific geologic formations and/or to progress inproducing desired degrees for fracture within a geologic formation,predetermined stimulation protocols may be used instead to simplifyoperations and/or lower costs.

In certain embodiments, frac diagnostic software and data to implementsensorless control via operating parameters (e.g., magnetic fieldstrength and polarity) of electromagnet 564, or to implement feedbackcontrol incorporating accelerometer 518, are conveniently stored andexecuted in a microprocessor (located, e.g., in controller 562). (See,e.g., U.S. Pat. No. 8,386,040 B2, incorporated by reference). See FIGS.5 and 6 of the '040 patent reference, for example, with theiraccompanying specification.

Note, however, that while certain of the electrodynamic controlcharacteristics of a tunable hydraulic stimulator may be represented inearlier devices, the tunable hydraulic stimulator's reliance onmechanical shock (i.e., generated by hammer strike and rebound) togenerate tuned vibration (i.e., vibration characterized by predeterminedmagnitude and/or frequency and/or PSD) imposes unique requirementsindicated by the dynamic responsiveness of certain stimulator elementsto other stimulator elements as described herein. Further, thepower/data cable 514, or an analogous communication medium or controllink, (see FIGS. 16 and 17) may extend to other hydraulic stimulatorsand/or to wellhead or other auxiliary equipment (see, e.g., FIG. 17)that may 1) power the hydraulic stimulator, 2) receive and transmitstimulation-related data, 3) coordinate stimulator operation (e.g.vibration phase, frequency, amplitude and/or PSD) with relatedequipment, and/or 4) modify driver-related frac diagnostic softwareprograms affecting tunable hydraulic stimulator operations.

Note also that in addition to individual applications of a tunablehydraulic stimulator, two or more such stimulators may operate in acombined tunable hydraulic stimulator array during a given stage offracking (e.g., in a temporarily isolated section or stage of horizontalwellbore). Section isolation in a wellbore may be accomplished withswell packers, which may function interchangeably in part as the tunableresilient circumferential seals described herein. A single tunablehydraulic stimulator or an interconnected tunable hydraulic stimulatorarray may be programmed in near-real time to alter stimulationparameters in response to changing conditions in geologic materialsadjacent to a wellbore. A record of such changes, together with results,guides future changes to increase stimulation efficiency.

In summary, the responsiveness of certain elements of a tunablehydraulic stimulator to other elements and/or to parameter relationshipsfacilitates operational advantages in various alternative stimulatorembodiments. Examples involving such responsiveness and/or parameterrelationships include, but are not limited to: 1) driver element 560comprises a field emission structure comprising anelectromagnet/controller 564/562 having cyclical magnetic polarityreversal characterized by a variable polarity reversal frequency; 2)longitudinal movement of hammer (or movable mass) element 540 isresponsive to the driver cyclical magnetic polarity reversal; 3)longitudinal movement of hammer element 540 striking, and reboundingfrom, fluid interface 520 may be substantially in phase with thepolarity reversal frequency to generate vibration transmitted by fluidinterface 520; 4) the driver element polarity reversal frequency may beresponsive to accelerometer 518's electrical signal (and thus responsiveto vibration sensed by accelerometer 518; 5) longitudinal movement ofhammer element 540 may be substantially in phase with the polarityreversal frequency; 6) longitudinal movement of hammer element 540striking, and rebounding from, fluid interface 520 has a characteristicrebound frequency which is the inverse of the hammer rebound cycle time;7) the hammer may rebound in phase with polarity reversal and; 8) thehammer rebound cycle time is a function of i) the cyclical magneticpolarity of driver element 560 and/or; ii) the moduli of elasticity ofhammer element 540 and fluid interface 520.

FIG. 17 schematically illustrates a 2-dimensional view of majorcomponents and interconnections of a tunable down-hole stimulationsystem 699, together with brief explanatory labels and comments oncomponent functions. As aids to orientation, a schematic wellbore isshown, including surface pipe connections with pumps. Hydraulic pathwaysare illustrated for transmitting broad-spectrum vibration to, andreceiving band-limited backscatter vibration from, down-hole geologicmaterial adjacent to the wellbore. The hydraulic pathways are shownpassing to and from geologic material via, e.g., a preformed casing slotor an explosively-formed casing perforation.

The tunable down-hole stimulation system 699 schematically illustratedin FIG. 17 is relatively sophisticated, employing several structures,functions and interactions that may appear in different inventionembodiments (but that need not appear in all invention embodiments) andare described in greater detail below. To improve clarity, certainstructures and functions inherent in the system of FIG. 17 areschematically represented in FIGS. 15-16. For example, references tospecific elements (e.g., hammer element 540 or fluid interface 520)should be understood with reference to FIGS. 15-16. Further, theillustration of tunable down-hole stimulator 638 in FIG. 17 should beunderstood as including a tunable vibration generator (labeled as such)which is analogous to the illustrated tunable hydraulic stimulator 599in FIGS. 15-16. So while a portion of tunable down-hole stimulator 638should be understood as schematically analogous to tunable hydraulicstimulator 599, it should also be recognized that stimulator 638represents a different (expanded in part) subset of structures andfunctions not represented in stimulator 599.

A first example of a tunable down-hole stimulation system is one of theembodiments schematically illustrated in portions of FIGS. 15-17. Theembodiment comprises at least one frac pump 688 for creating down-holehydraulic pressure, together with at least one tunable down-holestimulator 638. Each stimulator 638 comprises a tunable vibrationgenerator (labeled in FIG. 17) for transmitting vibration hydraulically,as well as a programmable controller 650 for creating a plurality ofcontrol signals and transmitting at least one control signal to eachsaid frac pump 688 and each said tunable down-hole stimulator 638.Additionally, each tunable down-hole stimulator 638 comprises at leastone accelerometer 518 for sensing vibration and for transmitting anelectrical signal derived therefrom. And the programmable controller 650is responsive to accelerometer 518 via the electrical signal derivedtherefrom.

A second example of a tunable down-hole stimulation system is one of theembodiments schematically illustrated in portions of FIGS. 15-17. Theembodiment comprises at least one frac pump 688 for creating down-holehydraulic pressure, together with at least one proppant pump 618connected in parallel with at least one frac pump 688 for addingexogenous proppant. The system further comprises at least one tunabledown-hole stimulator 638, each stimulator 638 comprising a tunablevibration generator (labeled in FIG. 17) having a characteristic reboundfrequency. A programmable controller 650 is included for creating aplurality of control signals and transmitting at least one controlsignal to each frac pump 688, each proppant pump 618, and each tunabledown-hole stimulator 638. Each tunable down-hole stimulator 638comprises at least one accelerometer 518 for detecting vibration and fortransmitting an electrical signal derived therefrom, and eachaccelerometer 518 is responsive to the characteristic rebound frequency.Finally, the programmable controller 650 is responsive to accelerometer518 via the electrical signal.

A third example of a tunable down-hole stimulation system is one of theembodiments schematically illustrated in portions of FIGS. 15-17. Theembodiment comprises a wellbore comprising a vertical wellbore, akickoff point, a heel, and a toe (all portions labeled in FIG. 17). Atleast one frac pump 688 creates down-hole hydraulic pressure in thewellbore, and at least one tunable down-hole stimulator 638 is locatedwithin the wellbore (and between the heel and toe as labeled in FIG.17). Each stimulator comprises a tunable vibration generator (labeled inFIG. 17), and a programmable controller 650 creates a plurality ofcontrol signals and transmits at least one control signal to each fracpump 688 and each tunable down-hole stimulator 638. Each tunabledown-hole stimulator 638 comprises at least one accelerometer 518 forsensing vibration and for transmitting an electrical signal derivedtherefrom, and the programmable controller 650 is responsive toaccelerometer 518 via the electrical signal.

An alternative embodiment included in the tunable down-hole stimulationsystem 699 of FIG. 17, for example, comprises at least one frac pump 688for creating down-hole hydraulic pressure. System 699 further comprisesat least one down-hole tunable hydraulic stimulator 638 for generationand transmission of broad-spectrum vibration, and for detection ofbackscatter vibration, stimulator 638 being hydraulically pressurized byfrac pump 688. A programmable controller 650 is linked to at least onefrac pump 688 and at least one tunable hydraulic stimulator 638 forcontrolling down-hole hydraulic pressure and vibration generation asfunctions of backscatter vibration sensed by one or more detectors on atleast one tunable hydraulic stimulator 638. Each tunable hydraulicstimulator 638 comprises a movable mass or hammer element 540 (see FIGS.15-16) which is movable via a field emission structure in the form of anelectromagnet/controller 562/564 to strike, and rebound from, a fluidinterface 520 (see FIGS. 15-16) for generating broad-spectrum vibration(see FIG. 17). At least one tunable hydraulic stimulator 638 detects thebackscatter vibration via an accelerometer 518 coupled to fluidinterface 520 (see FIGS. 15-16). An electric signal derived fromaccelerometer 518 is carried via link 516, link 514 and at least oneadditional link 654 (labeled in FIG. 17) to programmable controller 650.The broad-spectrum vibration indicated in FIG. 17 is characterized by avibration spectrum having a predetermined power spectral density, andprogrammable controller 650 (see FIG. 17) alters the predetermined powerspectral density during the course of stimulation as a function of thebackscatter vibration.

The alternative embodiment of the tunable stimulation system 699described above may be further described as follows: tunable down-holehydraulic stimulator 638 comprises a hollow cylindrical housing 590having a longitudinal axis, a first end 594, and a second end 592, firstend 594 being closed by fluid interface 520 for transmitting andreceiving vibration, and fluid interface 520 comprising at least oneaccelerometer 518 for producing an accelerometer signal representingvibration transmitted and received by fluid interface 520. A driverelement 560 reversibly seals second end 592, and driver element 560comprises a field emission structure comprising anelectromagnet/controller 562/564 having cyclical magnetic polarityreversal characterized by a variable polarity reversal frequency.

The alternative embodiment of the tunable stimulation system 699 mayadditionally comprise at least one temperature sensor (labeled in FIG.17). Down-hole hydraulic pressure may be sensed (as labeled in FIG. 17)and transmitted as a pressure signal derived therefrom. Programmablecontroller 650 (through change in one or more of the control signals itproduces) is responsive to the pressure signal when present. Pressuremay analogously be controlled as a function-in-part of both temperatureand backscatter vibration sensed at tunable down-hole stimulator 638.And predetermined power spectral density may similarly be altered as afunction-in-part of both temperature and backscatter vibration sensed attunable down-hole stimulator 638.

In the above embodiments, a field emission structure may be responsiveto at least one control signal. Such responsiveness to at least onecontrol signal is achieved, e.g., by emitting one or more electricand/or magnetic fields which are functions of at least one controlsignal as sensed by the field emission structure through change in oneor more field emission structure electrical parameters. Thus, a tunablevibration generator may have a predetermined PSD which isdependent-in-part on one or more field emission structures that arethemselves responsive to at least one control signal.

What is claimed is:
 1. A method for tuning a down-hole stimulationsystem, the method comprising providing at least one frac pumpmaintaining a fluid environment in a wellbore, said fluid environmentcontacting geologic material adjacent to said wellbore; providing atunable down-hole stimulator in said wellbore, said stimulatorcomprising a vibration generator responsive to a periodic control signalby transmitting to said geologic material, via said fluid environment, avibration burst simultaneously comprising a plurality of vibrationfrequencies; and providing a programmable controller, said programmablecontroller transmitting each said periodic control signal to saidtunable down-hole stimulator; wherein said vibration generator comprisesan electromagnetic driver, a movable hammer, and a fluid interface, saidmovable hammer being responsive to said electromagnetic driver forperiodically striking said fluid interface, and rebounding therefromduring an adjustable rebound cycle time, to transmit one said vibrationburst to said geologic material via said fluid environment; wherein saidelectromagnetic driver is responsive to each said periodic controlsignal; wherein each said vibration burst has an adjustable powerspectral density, each said adjustable power spectral density beingresponsive to one said adjustable rebound cycle time; wherein each saidvibration burst excites induced geologic resonance vibration in saidgeologic material; wherein said fluid interface comprises at least oneaccelerometer for detecting each said vibration burst and detectingtime-shifted backscatter vibration energy from said induced geologicresonance vibration, said at least one accelerometer transmitting anaccelerometer signal derived from detected vibration; wherein each saidadjustable rebound cycle time is responsive to one said accelerometersignal for tuning said down-hole stimulation system; and whereinshortening one said adjustable rebound cycle time up-shifts one saidadjustable power spectral density.
 2. A method for tuning a down-holestimulation system, the method comprising providing at least one fracpump maintaining a fluid environment in a wellbore, said fluidenvironment contacting geologic material adjacent to said wellbore;providing a tunable down-hole stimulator in said wellbore, saidstimulator comprising a vibration generator responsive to a periodiccontrol signal by transmitting to said geologic material, via said fluidenvironment, a vibration burst simultaneously comprising a plurality ofvibration frequencies; and providing a programmable controller, saidprogrammable controller transmitting each said periodic control signalto said tunable down-hole stimulator; wherein said vibration generatorcomprises an electromagnetic driver, a movable hammer, and a fluidinterface, said movable hammer being responsive to said electromagneticdriver for periodically striking said fluid interface, and reboundingtherefrom during an adjustable rebound cycle time, to transmit one saidvibration burst to said geologic material via said fluid environment;wherein said electromagnetic driver is responsive to each said periodiccontrol signal; wherein each said vibration burst has an adjustablepower spectral density, each said adjustable power spectral densitybeing responsive to one said adjustable rebound cycle time; wherein eachsaid vibration burst excites induced geologic resonance vibration insaid geologic material; wherein said fluid interface comprises at leastone accelerometer for detecting each said vibration burst and detectingtime-shifted backscatter vibration energy from said induced geologicresonance vibration, said at least one accelerometer transmitting anaccelerometer signal derived from detected vibration; wherein each saidadjustable rebound cycle time is responsive to one said accelerometersignal for tuning said down-hole stimulation system; and whereinlengthening one said adjustable rebound cycle time down-shifts one saidadjustable power spectral density.
 3. A down-hole stimulation systemcomprising a frac pump maintaining a down-hole fluid environment in awellbore, said fluid environment contacting geologic material adjacentto said wellbore; a down-hole stimulator for transmitting, via saidfluid environment and in response to a periodic control signal, avibration burst having an adjustable power spectral density, each saidvibration burst exciting induced geologic resonance vibration in saidgeologic material; and a programmable controller for transmitting eachsaid periodic control signal to said down-hole stimulator; wherein saiddown-hole stimulator comprises a hollow cylindrical housing having alongitudinal axis, a first end, and a second end, said first end beingclosed by a fluid interface for transmitting and receiving vibration viasaid fluid environment, said fluid interface comprising at least oneaccelerometer for producing an accelerometer feedback signalrepresenting vibration transmitted and received by said fluid interface;a driver element reversibly sealing said second end; and a hammerelement longitudinally movable within said housing between said driverelement and said fluid interface, said hammer element being responsiveto said driver element for periodically striking said fluid interface,and rebounding therefrom during an adjustable rebound cycle time, totransmit one said vibration burst comprising a plurality of transmittedvibration frequencies to said geologic material via said fluidenvironment; wherein said driver element comprises anelectromagnet/controller having cyclical magnetic polarity reversalcharacterized by a variable polarity reversal frequency; wherein saidvibration transmitted and received by said fluid interface includes bothtransmitted vibration energy and time-shifted backscatter vibrationenergy from said induced geologic resonance vibration; whereinlongitudinal movement of said hammer element is responsive to saidcyclical magnetic polarity reversal; wherein longitudinal movement ofsaid hammer element striking, and rebounding from, said fluid interfaceis in phase with said variable polarity reversal frequency; wherein eachsaid adjustable power spectral density is responsive to one saidadjustable rebound cycle time; and wherein shortening one saidadjustable rebound cycle time up-shifts one said adjustable powerspectral density.
 4. A down-hole stimulation system comprising a fracpump maintaining a down-hole fluid environment in a wellbore, said fluidenvironment contacting geologic material adjacent to said wellbore; adown-hole stimulator for transmitting, via said fluid environment and inresponse to a periodic control signal, a vibration burst having anadjustable power spectral density, each said vibration burst excitinginduced geologic resonance vibration in said geologic material; and aprogrammable controller for transmitting each said periodic controlsignal to said down-hole stimulator; wherein said down-hole stimulatorcomprises a hollow cylindrical housing having a longitudinal axis, afirst end, and a second end, said first end being closed by a fluidinterface for transmitting and receiving vibration via said fluidenvironment, said fluid interface comprising at least one accelerometerfor producing an accelerometer feedback signal representing vibrationtransmitted and received by said fluid interface; a driver elementreversibly sealing said second end; and a hammer element longitudinallymovable within said housing between said driver element and said fluidinterface, said hammer element being responsive to said driver elementfor periodically striking said fluid interface, and rebounding therefromduring an adjustable rebound cycle time, to transmit one said vibrationburst comprising a plurality of transmitted vibration frequencies tosaid geologic material via said fluid environment; wherein said driverelement comprises an electromagnet/controller having cyclical magneticpolarity reversal characterized by a variable polarity reversalfrequency; wherein said vibration transmitted and received by said fluidinterface includes both transmitted vibration energy and time-shiftedbackscatter vibration energy from said induced geologic resonancevibration; wherein longitudinal movement of said hammer element isresponsive to said cyclical magnetic polarity reversal; whereinlongitudinal movement of said hammer element striking, and reboundingfrom, said fluid interface is in phase with said variable polarityreversal frequency; wherein each said adjustable power spectral densityis responsive to one said adjustable rebound cycle time; and whereinlengthening one said adjustable rebound cycle time down-shifts one saidadjustable power spectral density.